MAFIC ROCKS of the PANTS LAKE INTRUSION and RELATED Ni–Cu–Co MINERALIZATION in NORTH–CENTRAL LABRADOR

Current Research (1999) Newfoundland Department of Mines and Energy Geological Survey, Report 99-1, pages 215-253

MAFIC ROCKS OF THE PANTS LAKE INTRUSION AND RELATED Ni–Cu–Co MINERALIZATION IN NORTH–CENTRAL LABRADOR

A. Kerr Mineral Deposits Section

ABSTRACT

The Mesoproterozoic Pants Lake intrusion (PLI) was emplaced passively into sulphide-bearing paragneisses and orthogneisses of the Churchill Province, immediately west of the Nain–Churchill boundary. In the four years since the Voisey's Bay discovery, the PLI has become a major focus for Ni–Cu exploration, and is now known to host widespread disseminated, and rarer massive, magmatic sulphide mineralization. The PLI represents the closest analogue to the Voisey's Bay troctolitic host rocks recognized in northern Labrador.

The three main units of the PLI are a fine-grained, variably layered olivine gabbro containing cumulus olivine, a massive, coarse-grained leucogabbro containing late interstitial olivine, and a black olivine gabbro that is texturally akin to the massive leucogabbro and locally difficult to discriminate from it. The coarse-grained unit appears to postdate the layered unit, but the relationship of the black olivine gabbro to other units is ambiguous. The PLI component intrusions are broadly sheet- like in geometry, and range in thickness from < 50 m to over 600 m. Their present geometry is complex, and may reflect the superposition of tilting and/or gentle folding on primary intrusive variations in thickness and anatomy. The mineralized basal contacts of intrusions lie within feasible exploration depths over large areas, and are far from fully explored.

Magmatic sulphide mineralization is ubiquitous near the basal contacts of the PLI component intrusions, and is commonly associated with distinctive, contaminated, sulphide-bearing mafic rocks that include textural equivalents of the "basal breccias" and "leopard troctolites" described from the Voisey's Bay deposit. Metal contents at 100 percent sulphide are, on average, lower than at Voisey's Bay, but Co is significantly higher at a given Ni content. There are indications of higher metal contents, up to 11.8% Ni and 9.7% Cu, in sulphide zones that may represent fractionated sulphide liquids. The PLI mineralized sequences appear to be intrusion-wide "stratigraphic" features, rather than being confined to feeder conduit systems, and their distribution and relationships to the intrusions may differ from the Voisey's Bay area. In both areas, the mineralized sequences record the emplacement of discrete sulphide- and fragment-bearing magma pulses into active, dynamic, mafic magma chambers, but the precise details of their evolution may differ. However, the PLI clearly demonstrates that the distinctive rock types and processes seen (and inferred) at Voisey's Bay are present elsewhere in Labrador.

INTRODUCTION and mafic host rocks since 1996. This article brings togeth- er observations and ideas developed over this period, and Mineral exploration in northern Labrador increased dra- integrates them with results from the exploration program. matically in 1995, following the Voisey's Bay discovery of Interpretations presented here are current (but still early) late 1994, but overall activity has declined since 1996. views, and will be refined and/or modified as additional However, the "South Voisey's Bay Project" of Donner information is gathered from continued exploration and Minerals and their joint venture partners progressively related research theses. However, in view of the wide inter- expanded from 1995 to 1998, reflecting positive indicators est in this project an overview of this type is timely. of economic-grade mineralization, and recognition that the geology has many intriguing similarities to the area around The study area is located in north–central Labrador, Voisey's Bay itself (e.g., Kerr and Smith, 1997; Fitzpatrick about 130 km south of Nain and 90 km west of Hopedale, et al., 1998). and forms part of a highland area north of the Adlatok (or Ugjoktok) River. Regionally, the area (Figure 1) is dominat- The Geological Survey of Newfoundland and Labrador ed by Paleoproterozoic gneisses of the Churchill Province, has been examining the regional geology, mineralization and is situated between two major Mesoproterozoic igneous

215 CURRENT RESEARCH, REPORT 99-1

Figure 1. General geology of northern Labrador, showing the Nain and Churchill provinces, and the Mesoproterozoic Nain and Harp Lake plutonic suites. The area of active exploration at the South Voisey's Bay project, and the Pants Lake intrusion, are indicated.

216 A. KERR complexes, i.e., the Nain Plutonic Suite (1350 to 1290 Ma) north to the Nain Plutonic Suite (although none are dated). and the Harp Lake Plutonic Suite (~ 1450 Ma). The Nain The youngest component of the latter is the Pants Lake Province–Churchill Province boundary zone lies in the east- intrusion consisting of an elongated belt of mafic plutons ern part of the area, but its characteristics in this region are running northwest–southeast through the centre of the area poorly known (Ryan, 1996). The gneisses of the Churchill (Figure 2). Province are intruded, over a wide area, by post-tectonic mafic plutonic rocks that collectively form the Pants Lake NAIN PROVINCE BASEMENT ROCKS intrusion (PLI), assigned to the Nain Plutonic Suite. The PLI hosts widespread Ni–Cu sulphide mineralization, and is the These are described by Thomas and Morrison (1991), main focus of this article. and were reexamined adjacent to the Nain–Churchill boundary. The rocks are dominated by banded, grey, tonalitic to Sulphide mineralization was first noted by Thomas and granodioritic orthogneiss; the banding partly reflects trans- Morrison (1991), who also initially delineated the mafic posed primary compositional variation and partly, the results rocks of the PLI. There was essentially no exploration prior of migmatization. Concordant bands of dark grey to black to late 1995, when Donner Resources acquired an interest in amphibolite represent either supracrustal remnants or trans- a large tract of land centred on anomalous Ni values in lake posed mafic dykes. No intense deformation or mylonitiza- sediment, which are spatially associated with the PLI mafic tion were observed in Archean gneisses adjacent to the rocks. Public-domain information released from the explo- Nain–Churchill boundary, but it is present in paragneiss ration program has been summarized in previous reports west of the boundary (see below). The Archean gneisses (Kerr and Smith, 1997, 1998; Kerr, 1998a), and is not contain small shear zones adjacent to, and subparallel with, repeated here. The 1998 program was the most extensive to the boundary. date, and included about 15 000 m of diamond drilling. Although no economic deposits have been discovered, CHURCHILL PROVINCE BASEMENT ROCKS Donner Minerals have stated their intention to mount a 1999 exploration program. Pelitic and Psammitic Paragneiss

This is the dominant unit within the Churchill Province REGIONAL GEOLOGY in the study area, as noted by Thomas and Morrison (1991). Subsequent company mapping has confirmed this pattern, The geology of the area is depicted in Figure 2. The but the gneisses are highly variable on an outcrop scale, and present outline and subdivision of the PLI is mainly derived mappable compositional variants are difficult to define. from mapping by Donner Minerals and Teck Corporation Paragneiss outcrops invariably contain up to 50 percent leu- (Wares, 1997; Fitzpatrick et al., 1998), whereas country cocratic granitoid gneiss and pegmatite, and granitoid gneiss rock units are taken from Hill (1982), Thomas and Morrison outcrops invariably contain numerous screens and zones of (1991) and company mapping. The current project is aimed paragneiss. Thus, delineation of "paragneiss" and mostly at understanding mineralization, and no systematic "orthogneiss" units is inherently subjective. The pattern mapping was conducted. However, key contacts and sec- illustrated in Figure 2 is derived mainly from company map- tions were examined, and areas outside the joint venture ping (Fitzpatrick et al., 1998), and modified from Thomas were mapped at a reconnaissance level. and Morrison (1991) in areas outside the exploration project. GEOLOGICAL FRAMEWORK Paragneisses are white- to pale-grey- or rusty-weather- The area includes four main "packages" of rocks. In the ing, and well banded. Banding is defined primarily by the east, quartzofeldspathic orthogneisses and associated alternation of granitic layers and biotite-rich bands, and like- supracrustal rocks are part of the Hopedale Block of the ly records repeated migmatization, and deformation. Well- Archean Nain Province. These are in presumed tectonic banded gneiss commonly passes along strike into nebulitic contact with gneisses and granitoid rocks of the Churchill material indicating, some late, static migmatization. The Province, which are dominated in this area by darker, restitic material consists of quartz, plagioclase, K- Paleoproterozoic supracrustal rocks. These are dominated feldspar, biotite, muscovite, garnet and sillimanite, indicat- by pelitic to psammitic paragneiss and leucocratic granitic ing amphibolite-facies conditions. Cordierite is locally orthogneiss, commonly intermixed on an outcrop scale. prominent, and likely present as a cryptic phase in most out- Undeformed Mesoproterozoic anorthosites, diorites and crops. Leucocratic zones commonly contain large garnets up granites occur in the north and south, and intrude both Nain to 2 cm in diameter, and have a "spotty" appearance. and Churchill province gneisses. Those in the south are Graphite is locally abundant in the restite, and many out- assigned to the Harp Lake Plutonic Suite, and those in the crops are rusty-weathering, suggesting that disseminated

217 CURRENT RESEARCH, REPORT 99-1

Figure 2. Simplified geological map of the Pants Lake intrusion and surrounding area. Geology largely derived from mapping by Donner Minerals and Teck Corporation, with information from Thomas and Morrison (1991), Hill (1982) and field work by the author. Note that some unit distributions, notably in the northern part of the North intrusion are presently based largely on drillhole information. Numbers on the margins of the figure are UTM coordinates in zone 20.

218 A. KERR sulphide is also present. The latter has been entirely oxi- lates containing interstitial to subophitic orthopyroxene, dized in many outcrops, but is noted also in diamond-drill commonly brown- weathering. A yellow–brown interstitial core. phase noted in some outcrops, particularly in the south, may be an iron-rich olivine. Although the anorthosites in the The structural pattern in the paragneisses is locally north and south belong to different plutonic suites, and are complex, with well-developed rootless and interference- likely of different ages, they are generally similar in appear- type folds, indicative of polyphase deformation (Thomas ance. Coarse-grained, plagioclase-rich plutonic rocks have and Morrison, 1991). Mylonitic zones having a north- been intersected in several deep drillholes collared adjacent west–southeast trend (parallel to regional trends) occur in to, or within, the PLI, and were originally thought to be part some outcrops, and deflect an earlier mineral fabric (with a of its massive leucogabbro unit (see below). However, these sinistral sense of motion) producing spectacular crenulated are now considered to be part of anorthositic plutons that outcrops. The regional structural pattern may mirror these predate the PLI (D. Fitzpatrick and P. Moore, personal com- outcrop-scale relationships, as foliations in the paragneiss munication, 1998), as they are intruded by dioritic rocks, unit fall into two groups, with broadly northwest–southeast diabase dykes and granitic veins, all of which are rare in PLI and east–west attitudes (Fitzpatrick et al., 1998). gabbros.

Granitoid Gneisses Ferrodiorite and Monzodiorite

Hill (1982) and Thomas and Morrison (1991) recog- These occur only in the northwest of the area (Hill, nized two types of granitoid gneiss within the area, and 1982), and were confirmed by company mapping (Figure 2); company mapping (Fitzpatrick et al., 1998) confirms this they are assigned to the Nain Plutonic Suite. Where fresh, subdivision. "Charnockitic" granitoid gneisses occur adja- these are typically medium-grained, grey–green rocks, but cent to the Nain–Churchill boundary, north and south of they are commonly deeply weathered and rusty. The plagio- Sarah Lake, and consist of medium- to coarse-grained, gra- clase-rich variants are similar in general appearance to many noblastic gneiss composed of quartz, plagioclase, orthopy- of the anorthosites. Dykes of medium-grained diorite roxene and hornblende, and lesser biotite and opaques. intrude neighbouring anorthosites, and rafts of anorthosite These rocks resemble "charnockite" or "enderbite" gneiss are present in the diorites, indicating that the diorites are reported from equivalent regions of the Churchill Province younger, which is a common pattern. Hill (1982) suggested to the north (e.g., Ryan, 1996). The more widespread type that many of the intermediate plutons had a broad sheet-like consists of garnetiferous granitoid gneiss and pegmatite, geometry, and this pattern is also known from the Nain area which is present in almost every paragneiss outcrop. In (Ryan, 1990). many cases, the granitic material is texturally variable, and likely represents local anatectic segregations. In addition to Quartz Monzonite, Syenite and Granite this pervasive outcrop-scale material, there are several regional granitoid gneiss units (Figure 2), also of probable The largest area of Mesoproterozoic granitoid rocks is anatectic origin. These are dominated by white, coarse- located between the Pants River and the Adlatok River grained, variably K-feldspar porphyritic, garnetiferous gran- (Figure 2), and forms part of the Arc Lake Granite, assigned itoid gneiss, locally showing well-preserved igneous tex- to the Harp Lake Plutonic Suite (Emslie, 1980; Thomas and tures. More commonly, these show a cataclastic texture, and Morrison, 1991). This is dominated by massive syenite and the contacts between some "orthogneiss" and "paragneiss" granite, commonly K-feldspar porphyritic, containing vari- areas contain mylonitic zones. ably developed mantled-feldspar ("rapakivi") textures. Primary mafic minerals include orthopyroxene and clinopy- MESOPROTEROZOIC PLUTONIC ROCKS roxene, and rarer fayalitic olivine; the latter is associated with rusty-weathering. Anorthositic rafts are present in some Anorthosite outcrops, and gneissic inclusions are also present, particularly near the northern contact. Mesoproterozoic granite also Anorthosite rocks assigned to the Nain and Harp Lake occurs northeast of Sarah Lake (Hill, 1982). plutonic suites occur in three main areas (Figure 2), two of which are adjacent to, or within, the outcrop area of the PLI. Pants Lake Intrusion (PLI) These anorthosite rocks present a problem, as many are superficially similar to coarse-grained, massive leucogabbro The PLI was defined as a separate entity by Fitzpatrick of the PLI, and the two are easily confused. Much of the et al. (1998), who used the term "Pants Lake Intrusive north intrusion of the PLI was grouped with Nain Plutonic Suite". In accordance with provisions of the stratigraphic Suite anorthosite rocks by Hill (1982). In all areas, the code, the term Pants Lake intrusion is employed here. U–Pb anorthosites are typical coarse- grained plagioclase cumu- geochronological studies (G.R. Dunning, confidential report

219 CURRENT RESEARCH, REPORT 99-1 to Donner Minerals, 1998) indicates that it should be includ- gabbro and coarse- grained, massive, leucogabbro. The for- ed with the Nain Plutonic Suite, rather than the Harp Lake mer is most abundant in the South intrusion, but also occurs Plutonic Suite. No details of the geochronology are present- in the lowermost parts of the North intrusion. The latter is ed here, but the PLI yields an age that falls within the age the dominant unit in surface outcrops of the North intrusion. range of troctolitic intrusions at Voisey's Bay, as reported by Melagabbro and peridotite forms a minor (but important) Amelin et al. (1997, 1998). The PLI is dominated by olivine unit in the South intrusion, so far encountered only in drill- gabbro of various types, described in detail in a following holes. Black olivine gabbro (commonly abbreviated to sim- section. ply "black gabbro") occurs in some parts of the North intrusion. This fine- to coarse-grained unit was previously Mafic Dykes described as troctolitic (Fitzpatrick et al., 1998), but is generally of gabbroic composition. This rock type is difficult to Mafic dykes are most commonly observed in plutonic distinguish from texturally similar massive leucogabbro, rocks of the Nain and Harp Lake plutonic suites. These must and the pattern in Figure 2 largely reflects drillhole infor- also cut gneissic basement rocks, but are less commonly mation. Two other rock types are locally associated with the seen in these units. Most are fresh, aphanitic to fine-grained external contacts of the PLI. Diabase is locally seen at lower diabase, and locally show alteration and shearing along their and upper contacts, where it records chilling effects. contacts. At least some have the east–northeast trend typical Intrusive breccia occurs in several areas at contacts, and of the Harp Dykes (Emslie, 1980), but there is likely more consists of angular to locally resorbed gneissic fragments in than one generation present. Mafic dykes are only rarely a fine-grained mafic matrix. In addition to these, several dis- observed to cut the PLI. A "flat-lying Harp dyke" described tinctive and unusual rock types occur in a thin zone of min- by Emslie (1980) south of Pants Lake is actually a gently eralized rocks at or just above basal contacts, particularly in dipping sheet of PLI gabbro. the North intrusion. These are collectively termed the mineralized sequences, as suggested by A.J. Naldrett (personal PANTS LAKE INTRUSION communication, 1998) and are discussed separately, in con- junction with mineralization. The PLI consists of several discrete mafic intrusions distributed through the centre of the area (Figure 2). The PRINCIPAL ROCK TYPES scattered geography, common rock types, and complex three-dimensional geometry of the PLI require that its Layered Fine-Grained Olivine Gabbro description be broken into two parts. The characteristics of the principal rock types, excluding mineralized rocks, are This unit is present in all the component intrusions, but presented first. This is followed by a discussion of the rela- may not have been derived from the same magma batch in tionships and geometry of the component intrusions, as different areas. The most extensive and informative expo- presently understood, and the possible links between them. sures are in the North intrusion, notably the NDT lobe. It is more poorly exposed in the South intrusion. In most other GEOGRAPHIC AND PETROLOGICAL areas, this unit is spatially associated (or part of) mineralized SUBDIVISIONS sequences, and characteristics are obscured by gossan development. Component intrusions of the PLI (Figure 2) contain similar rock types, and many are "stratigraphically" similar, North of Pants Lake, the unit displays the orange–red- but there are also important variations in their character and weathering typical of olivine-rich rocks. On fresh surfaces, anatomy. From south to north, the main subdivisions are it is a grey–green to dark grey, fine- to medium-grained (1 here termed the South intrusion, the Mineral Hill intrusions, to 4 mm), granular rock, consisting of olivine, plagioclase, the Worm intrusion and the North intrusion. For descriptive subophitic clinopyroxene, and minor magnetite and biotite. purposes, the North intrusion is subdivided into the NDT Layering is locally well developed on vertical cliff faces lobe, Happy Face lobe and Taheke Lake lobe (Figure 2). (Plate 1a), but is more difficult to see on flat outcrop sur- There are also two smaller intrusions entirely contained faces, as it is subhorizontal to gently dipping. Layering is within the granites of the Harp Lake Plutonic Suite; these invariably subtle, and appears in drill core as only slight are termed the Doughnut intrusion and the Cartaway intru- variations in colour and grain size. In some areas, the unit sion. The latter lies outside the Donner Minerals joint ven- develops a characteristic "speckly" texture due to the pres- ture project. ence of small (2 to 4 mm) plagioclase phenocrysts. The unit is monotonous, and subtle compositional and textural varia- The PLI includes several rock units, but these are not tions are commonly visible only in drill core (Plate 1d). The necessarily all present in each geographic subdivision. The fine-grained, layered, olivine gabbro of the South intrusion two most abundant units are fine-grained, layered, olivine is identical in appearance. Fine-grained olivine gabbro seen

220 A. KERR

Plate 1. Fine-grained, layered olivine gabbro unit of the Pants Lake intrusion. (A) Primary layering, defined by variations in mafic mineral content, with local "crossbedding" in centre of photo, NDT lobe, North intrusion. (B) Knobbly weathering texture indicating the presence of clinopyroxene oikocrysts in olivine-rich matrix, Mineral Hill intrusion. (C) Drill-core intersection showing veins of coarse-grained leucogabbro unit cutting fine-grained gabbro, hole 98-112. (D) Examples of textural variations in drill-core samples of the fine-grained unit. Note gneissic fragment in core sample at lower right. in the Mineral Hill, Cartaway and Worm intrusions, and clinopyroxene crystallized late. Magnetite and biotite are locally near the base of the North intrusion, shows a charac- also late-crystallizing phases. The crystallization sequence teristic "knobbly-weathering" texture (Plate 1b). This is indicates that many of these rocks are plagioclase–olivine caused by large (8 to 15 mm) poikilitic clinopyroxenes, set cumulates, but the range in modal compositions is limited, in a more recessive olivine-bearing groundmass. This tex- except locally in the South intrusion. ture is similar in many respects to the distinctive "leopard- Melagabbro and Peridotite texture" developed in parts of the mineralized sequence. These rock types occur in four drillholes in the South In thin section, fine-grained, layered olivine gabbro unit intrusion, where they form part of a mafic cumulate consists of olivine (30 to 60 percent), plagioclase (40 to 60 sequence near its basal contact. They were originally recog- percent; typically An50–An55) and clinopyroxene (5 to 30 nized by H. MacDonald and A.J. Naldrett. In drill core, they percent), along with minor amounts of red biotite, mag- are dark green, variably serpentinized, medium- to coarse- netite, and serpentine (after olivine). Orthopyroxene is pres- grained rocks consisting mainly of olivine, clinopyroxene ent only rarely, as thin rims on scattered olivine grains. A and interstitial plagioclase. Most examples contain at least few examples are true troctolites, but most are classified as 10 to 20 percent plagioclase, and are probably most accu- olivine gabbro. The unit displays a common texture, in rately described as melagabbro. The contacts between this which olivine is granular, plagioclase is lath-like, and rock type and associated layered olivine gabbro are abrupt clinopyroxene is interstitial (Plate 2a), subophitic or (more but gradational, suggesting that they are closely related. rarely) poikilitic (Plate 2b). Small olivine grains are typical- They are also locally interlayered with fine-grained olivine ly included in plagioclase laths. The texture indicates that gabbro. Two samples examined in thin section showed sig- olivine crystallized first, followed by plagioclase, and that nificant serpentinitic alteration.

221 CURRENT RESEARCH, REPORT 99-1

Plate 2. Petrography of fine-grained, layered olivine gab- Plate 3. Coarse-grained massive leucogabbro of the Pants bro. (A) Typical texture, consisting of plagioclase laths, Lake intrusion. (A) Typical field appearance, Worm intru- granular, cumulus olivine and late, clinopyroxene, PPL. (B) sion, note interstitial habit of all mafic minerals. (B) Typical Porphyritic variant, also showing development of clinopy- drill-core samples. The upper core sample is altered adja- roxene oikocrysts, XP. Boundaries of individual clinopyrox- cent to the upper contact, and the lower sample is a more ene oikocrysts are shown by white lines. Ol – olivine, Pl – typical fresh, seriate-textured variety. plagioclase, Cpx – clinopyroxene, PPL – plane-polarized light, XP – crossed polarizers. colours, and show variable alteration to amphibole and/or chlorite. The most leucocratic variants resemble Nain or Parts of the melagabbro and peridotite unit host signif- Harp Lake plutonic suite anorthosites. Many outcrops were icant disseminated magmatic sulphides, and they form part initially mapped as gabbronorite, as the contrasting mafic of the South intrusion mineralized sequence. The unit also minerals were assumed to be pyroxenes on the basis of their contains scattered and variably digested gneissic fragments. habit; however, most examples are actually olivine gabbro. The mineralization is discussed in a later section. In thin section, the best-preserved examples consist of Massive Coarse-Grained Leucogabbro plagioclase (60 to 80 percent), olivine (5 to 20 percent) and clinopyroxene (10 to 20 percent), with lesser amounts of Outside the South intrusion, massive, coarse-grained, biotite and magnetite (Plate 4a). Modal analysis is difficult, leucogabbro is the dominant rock type on surface. It is a as the mafic minerals commonly form large, single, optical- coarse-grained and homogeneous rock having a characteris- ly continuous crystals, and individual thin sections are troc- tic pale-grey- to white-weathering colour, and a variably tolitic, or olivine-free. Drill-core examination suggests that developed seriate to porphyritic texture (Plate 3a,b). olivine is generally less abundant than clinopyroxene, and Plagioclase crystals are commonly in the 1 to 2 cm range, most rocks are olivine gabbro. The larger plagioclase crys- and form stubby prisms, but the unit locally becomes tals show prominent zoning. Plagioclase compositions are extremely coarse grained, particularly in the Worm intrusion more variable than in the fine-grained gabbro, but have a (Plate 3a). Mafic minerals show an interstitial to subophitic similar maximum of around An55. Olivine, locally, is unusu- habit, have contrasting green- and brown-weathering al pale grey, probably caused by fine iron-oxide dust, and

222 A. KERR

grained amphibole-rich material surrounded by coarser hornblende.

Black Olivine Gabbro

This unit is presently known only from the northern part of the North intrusion, where closely spaced drilling has been conducted. Its characteristics are best known from drill core, where it is a medium- to coarse-grained dark grey to black rock that superficially appears very mafic. Its appearance is deceptive, as it contains over 60 percent dark-weathering plagioclase, with the remainder consisting mostly of olivine and purple–bronze clinopyroxene. Plagioclase is commonly prismatic and well-twinned, in contrast to the more equant habit typical of the massive leucogabbro (Plate 5a). Large, optically continuous olivine and clinopyroxene crystals displaying spectacular poikilitic textures, are

Plate 4. Petrography of coarse-grained, massive leucogabbro. (A) Typical texture in a fresh sample, showing interstitial to oikocrystic habit of olivine and clinopyroxene, PPL. (B) Altered variant showing extensive retrogression of mafic minerals (to actinolite, chlorite and serpentine) but reten- tion of original textural relationships, PPL. Pl – plagioclase, Ol - olivine, Am – amphibole, after clinopyroxene, Se – serpentinized olivine, Cpx - clinopyroxene, (photos in PPL – plane-polarized light). clinopyroxenes are commonly purplish and weakly pleochroic, suggesting that they are titaniferous. Thin reaction and/or alteration rims of orthopyroxene, surrounded by amphibole, are seen around olivines in some samples. Both olivine and clinopyroxene are interstitial to subophitic in habit, indicating that they crystallized late, in contrast to early olivine of the layered olivine gabbro unit (Plate 4a).

Altered massive leucogabbro is noted in drillholes that Plate 5. Characteristics of black olivine gabbro unit of the preserve an upper "cap" of gneissic country rocks. Pants Lake intrusion. (A) Typical drill-core samples of Clinopyroxene and olivine are extensively altered to sec- coarse-grained and medium-grained varieties. Despite their ondary amphibole, serpentine and iron oxide (Plate 4b), but dark colour, these samples are plagioclase-rich. (B) Large the retrogression diminishes downhole from the upper con- poikilitic olivine crystal in thin section; note similarity to tact. The upper regions of the massive leucogabbro unit also coarse-grained leucogabbro texture of Plate 4. Ol – olivine, contain rounded, very fine-grained inclusions of unknown Pl – plagioclase, Cpx – clinopyroxene. Photo in PPL origin; in thin section, these consist or extremely fine- (plane-polarized light).

223 CURRENT RESEARCH, REPORT 99-1 common in many examples. There are marked grain-size at the contact itself. However, diabase-like units observed in variations within this unit, and finer grained examples are gneisses below the intrusions have a greenish, altered black diabase-like rocks that contain large poikilitic olivines appearance, and contain extensive hornblende, most of and pyroxenes. Although some of the finer grained types are which appears primary. These are interpreted as older minor superficially similar to the layered olivine gabbro, their tex- intrusions that predate the emplacement of the PLI. Diabase ture is distinctly different in thin section. is only rarely observed within the PLI away from contact regions, which is consistent with the rarity of diabase dykes In drill core, black gabbro may show a blotchy appear- in the field. However, there are several intervals of diabase ance, probably indicating variable secondary effects, and the and very fine-grained gabbro in deep holes completed in the lighter coloured regions resemble the massive grey leuco- South intrusion. These appear to have sharp contacts, imply- gabbro. It is also texturally similar to the massive leucogab- ing that they postdate the dominant layered olivine gabbro bro, and contains the same late, interstitial, olivine and in this area. In thin section, all of these rocks are typical dia- clinopyroxene (Plates 4a, 5b). In surface outcrops, the dis- bases, consisting of finely intergrown plagioclase, clinopy- tinction between the two units is very difficult. Present roxene and olivine. They commonly have a turbid appear- information suggests that the black olivine gabbro occurs on ance due to dispersed iron oxide and alteration. It is com- surface in much of the area northeast of Taheke Lake, but its monly difficult to estimate the amount of olivine in these precise outline is approximate. In thin section, the coarse- fine-grained rocks, but it is usually present. grained black gabbro is closely similar to typical massive leucogabbro. The dark colour exhibited by plagioclase Intrusive Breccias appears to be related to numerous tiny iron-oxide inclusions. The finer grained variants are spectacular rocks in which Locally spectacular intrusive breccias (not shown in both olivine and clinopyroxene form poikilitic crystals; their Figure 2) are developed around the North intrusion, and also texture resembles that of a coarse diabase. in some gabbro outliers near the South intrusion. Typical examples contain angular to rounded blocks of orthogneiss Given the similarities between the coarse-grained, mas- and lesser metasedimentary gneiss. The matrix to the blocks sive, leucogabbro and the black olivine gabbro, their status may contain minor sulphide, and in one of the southern as discrete units has been keenly discussed. The author was examples appears vesicular, and possibly subvolcanic, as initially reluctant to separate them, but subsequently agreed noted by Thomas and Morrison (1991). Inclusions of with Fitzpatrick et al. (1998) after examining several key graphite are also recorded at this, and other localities. In the 1998 drillholes. These intersected both units, indicating that North intrusion, intrusive breccias appear to be developed they are indeed distinct, and demonstrate that they have dif- locally at the contacts of both layered olivine gabbro and ferent relationships to the fine-grained, layered olivine gab- massive leucogabbro, representing lower and upper contacts bro. The black olivine gabbro sits beneath the fine-grained respectively (Fitzpatrick et al., 1998). Although there is unit, whereas the massive leucogabbro sits above it. local assimilation and contamination in these breccia units, However, this relationship is not clearly reflected in the out- the gneiss fragments do not show the reaction textures crop pattern of the three units, as defined by mapping to date observed in drill core from the mineralized sequence and (Figure 2). most breccias are sulphide-poor. The xenolith population appears to be mostly of local origin, and had only a short Diabase "residence time" in the magma. These breccias are regarded as distinct from complex, sulphide-bearing, breccia-like Diabase occurs near the basal contacts of PLI intrusions rocks associated with the mineralized sequences. in several drillholes, and is a chilled version of overlying fine-grained gabbro, with which it is gradational. Although GEOMETRY, STRATIGRAPHY AND most of these chilled zones are close to, or within, the min- RELATIONSHIPS eralized sequence, the spatially associated gabbro is normal- ly sulphide-free, and is texturally akin to the layered olivine Diamond drilling since 1995 has shown that the PLI gabbro. In hole SVB-97-56 (near the Happy Face showing, component intrusions have complex geometries, much of Figure 2) a similar diabase unit is developed at the upper which would be difficult to infer from surface patterns contact of the massive leucogabbro unit, and grades down- alone. This section discusses their geometry, stratigraphy ward into it. and geological relationships, using summary maps and cross-sections (Figures 3, 4 and 5). Cross-sections all incor- Diabase also cuts the host gneisses in drill core, nor- porate a mild (200 percent) vertical exaggeration in order to mally close to the contacts of the PLI. Some examples prob- adequately represent the geology, and true unit thicknesses ably represent a chilled facies, which may not be preserved and dip angles are slightly distorted.

224 A. KERR

South Intrusion and 36 near Mineral Hill (Figure 3d). Each mafic interval in hole 98-110 contains an upper section of coarse-grained Only the northern portion of this intrusion (south of the massive leucogabbro, and has a thin mineralized sequence Pants River valley; Figures 2 and 3) is discussed here. This near its base. The upper part of hole 91 also contains a mafic is dominated by fine- to medium-grained, variably layered unit containing basal mineralization, but there is no sign of olivine gabbro. The coarse-grained massive leucogabbro the middle mafic unit from hole 94 (Figure 3a). unit occurs only as short intervals with sharp contacts in drill core, which have the appearance of intrusive veins or sheets. Shallow 1996 drilling illustrates intrusive geometry in Five deep vertical drill holes (SVB-97- 79, 86, 89, 94 and the area of the Mineral Hill showing (Figure 3d). The pat- 98-100) penetrate the intrusion, and four reached its basal tern can be interpreted as a single sheet-like body, which has contact (Figure 3b,c). In holes 79 and 86, peridotite and been gently folded (or displaced by a fault) or as two sepa- melagabbro are interlayered with fine-grained olivine gab- rate sheet-like intrusions. Given the evidence from hole 110, bro, and represent a basal mafic cumulate sequence, located the latter interpretation is preferred here (Figure 3d). 500 to 600 m below the surface. Identical cumulate rocks Although there are some uncertainties in correlation, the occur close to surface in hole 100, and a thin intersection of Mineral Hill intrusions clearly represent stacked subhori- sulphide-bearing gabbro at the very top of hole 89 probably zontal to gently west-dipping sheets sitting structurally represents this horizon. The four basal contacts lie on a above the South intrusion. The middle mafic section in hole plane striking at about 010E, and dipping west at about 15E. 94 (Figure 3b,c) may represent part of the conduit system The gabbro has a continuous true thickness of about 600 m that originally linked them to the South intrusion, but there in the area of hole 79, but hole 94 (abandoned at 501 m) con- is no evidence within this of the coarse-grained massive tains a central intersection of gneisses, separating two gab- leucogabbro seen in the higher intrusions. broic units, the upper of which hosts basal mineralization. At the equivalent stratigraphic level in hole 79, minor sulphide The Worm Intrusion mineralization is associated with thin screens of older plutonic rocks. The upper gabbroic sections in holes 79 and 86 The Worm intrusion is probably the least understood contain a higher density of diabase dykes compared to the component of the PLI. The intrusion forms a positive- lower sections. These contrasts suggest that the South intru- weathering ridge trending at about 140E, and the outcrop sion may itself be composite, and record two or more mag- pattern suggests that it is unlikely to be a subhorizontal matic influxes. The basal contact in hole 79 preserves a fine- sheet. The nature of the southern contact, where it approach- to medium-grained, barren gabbro that grades downward es the Mineral Hill and South intrusions, is obscured by drift into a chilled zone. This provides a possible sample of the in the Pants River valley. In the north, it appears to merge initial magma that entered the chamber, which solidified directly into the North intrusion southeast of the Happy Face prior to the formation of the overlying mafic cumulates. showing (Figure 2). The dominant rock type in the Worm intrusion is coarse-grained, massive leucogabbro which The western subsurface limit of the South intrusion is locally is very-coarse grained (e.g., Plate 3a). There are local not defined. It should be present in the lower part of the deep exposures, particularly on the west side, of fine-grained drillhole SVB-97-91 (Figure 3b), but this intersected older olivine gabbro with poikilitic clinopyroxene. Sulphides Harp Lake Plutonic Suite anorthosites, cut by ferrodiorites, occur locally at the north end, where a crude layering in a diabase and granite. However, if the South intrusion thins small showing (probably defined by disk-like mafic inclu- slightly downdip, or its dip increases by as little as 5 to 10E, sions, as discussed later) is apparently truncated along strike its top would lie below the base of hole 91 at 1099 m. The by the massive leucogabbro. nature and attitude of the southern contact is unknown at present. About 1 km north of the Pants River, holes 96-29, 30 and 31 demonstrate that the Worm intrusion consists of a Mineral Hill Intrusions sheet, some 150 m thick, striking at 135E, and dipping some 35E to the northeast (Figure 3e). This is a near-surface atti- Figure 3 illustrates geometric relationships between the tude only, and it may steepen at depth. The sheet consists South intrusion and thinner sheet-like intrusions that host largely of coarse-grained leucogabbro, overlying a mineral- sulphide mineralization, including the Mineral Hill show- ized sequence. Other drillholes farther to the north do not ing. Hole 98-110 intersected three discrete mafic intrusions constrain the attitude of the intrusion. Locally, the Worm (Figure 3c). The lower two probably correlate with the two intrusion is very thin; in hole 96-15 (located north of Figure upper mafic intersections in hole 94, and the uppermost unit 3a), the entire sequence from coarse leucogabbro to footwall probably correlates with mineralized gabbro in holes 96-35 gneiss is contained in less than 50 m.

225 CURRENT RESEARCH, REPORT 99-1

Figure 3. Geometric views of the South intrusion, Mineral Hill intrusions and Worm intrusion. (A) Map showing outline, drillhole locations and section planes. (B) Approximately east–west cross-section of South intrusion (B-B'). (C) Approximately northwest–southeast cross-section of South intrusion (C-C'). (D) Approximately east- northeast–west-southwest cross-section of Mineral Hill area (D-D'). Distribution of mineralized sequence is partly schematic. (E) Northeast–southwest cross-section of the Worm intrusion (E-E') Cross-sections include 200 percent vertical exaggeration for clarity.

Cartaway and Doughnut Intrusions North Intrusion

These two small bodies have not been drilled, but the The North intrusion is the most complex component outcrop patterns and geological contact relationships sug- body of the PLI. Its surface and subsurface area is large, and gest that both are sheet-like. The Cartaway intrusion was the only closely spaced drilling is at the northern tip (Figure originally noted by Emslie (1980) as a "flat-lying" Harp 4). Elsewhere, it is poorly known at depth, and only a tiny Dyke. It contains an upper massive leucogabbro and a lower fraction of its basal contact has actually been tested. For pur- fine- grained gabbro, locally with poikilitic clinopyroxene. poses of description, the intrusion is logically subdivided The total thickness is approximately 50 to 70 m, and it dips into three "lobes", which are here termed the NDT lobe, very gently northward. The terminations of the unit (Figure Happy Face lobe and Taheke Lake lobe. 2) are limits of mapping only, and extensions of it may be present elsewhere south of Pants Lake. The Doughnut intrusion has not been examined by the author.

226 A. KERR

Figure 3. Continued.

227 CURRENT RESEARCH, REPORT 99-1

Figure 4a. Geometric views of the North intrusion. (A) Map showing outline, drillhole locations, and section planes. (B) NDT lobe, north–south cross-section (B-B'). (C) Happy Face lobe, north–south cross-section (C-C'). (D) Happy Face lobe, cross- section near GG showing (D-D'). (E) Happy Face lobe, northeast–southwest cross-section (E-E'). (F) Happy Face lobe, northwest–southeast cross-section (F-F'). (G) Taheke Lake lobe, east–west cross-section including Northern Abitibi drilling area (G-G'). (H) Taheke Lake lobe, southwest–northeast cross-section (H-H'). Cross-sections include 200 percent vertical exaggeration for clarity.

NDT Lobe north of Taheke Lake, because there is no sign of a thick intrusion in hole 98-111. In the west, the intrusion becomes This occupies the area between Pants Lake and Taheke narrower, and its basal contact must descend from about 500 Lake (Figures 2 and 4a), and includes the "NDT" surface m elevation at surface, to only 31 m above sea level in hole showings. A north–south cross-section from the surface 97-82, which implies a steeper northward dip of 25 to 30E. showings to Taheke Lake (Figure 4b) indicates a north-dipping intrusion at least 400 m thick, with about 100 m of mas- All of the drillholes in this area show similar rock types sive leucogabbro underlain by up to 300 m of fine- to medi- and relationships. The relationship between fine-grained um-grained, layered, olivine gabbro. A basal mineralized olivine gabbro and massive leucogabbro is commonly sequence, locally up to 30 to 40 m thick, projects to the sur- ambiguous, but short intervals of coarse-grained gabbro face showings (Figure 4b). The basal contacts of drillholes within the former may be intrusive veins or sheets. The lay- define an approximate plane, striking at 060E, and dipping ered olivine gabbro locally develops a "speckly" porphyrit- at about 12E to the northwest. It is not clear what happens ic appearance and, near its base, is almost as coarse-grained

228 A. KERR

Figure 4b. Geometric views of the North intrusion. (A) Map showing outline, drillhole locations, and section planes. (B) NDT lobe, north–south cross-section (B-B'). (C) Happy Face lobe, north–south cross-section (C-C'). (D) Happy Face lobe, cross- section near GG showing (D-D'). (E) Happy Face lobe, northeast–southwest cross-section (E-E'). (F) Happy Face lobe, northwest–southeast cross-section (F-F'). (G) Taheke Lake lobe, east–west cross-section including Northern Abitibi drilling area (G-G'). (H) Taheke Lake lobe, southwest–northeast cross-section (H-H'). Cross-sections include 200 percent vertical exaggeration for clarity. as the upper unit, but retains granular, cumulus olivine. area, the North intrusion ranges from a thin subhorizontal Some of these rocks are troctolitic, and may represent zones sheet, to a northeast-dipping sheet from 100 to 200 m in of olivine accumulation. A similar rock occurs just above the thickness (Figure 4). At the GG showing, there is a typical mineralized sequence in hole 96-04 (Figure 4a) implying sequence of coarse-grained massive leucogabbro sitting that this is a persistent feature. In the NDT lobe, the miner- above a mineralized sequence (Figure 4d). One of these alized sequence is always separated from the massive leuco- holes (96-02) indicates a second gabbro unit at slightly gabbro by a thick sequence of layered olivine gabbro, in deeper levels. This is the only evidence at present in the contrast to the Happy Face lobe. A fine-grained, dark rock North intrusion for "stacked" sheet-like intrusions akin to having a coarse diabasic texture, and poikilitic olivine, was those documented at Mineral Hill. A north-northwest–south- noted near the base of hole 97-63, and may correlate with southeast section (Figure 4c) suggests that the gabbro dips the black olivine gabbro seen in the Taheke Lake lobe. gently (about 5E) to the north, achieving a thickness of about 200 m, most of which is massive leucogabbro (Figure 4e). Happy Face Lobe However, a southwest–northeast cross-section from the Happy Face showing to hole 96-06 shows a northeast dip of The Happy Face lobe is separated from the NDT lobe about 12E, and also possibly gentle folding or undulation by a region of gneisses, locally overlain by small gabbro (although minor faulting could also explain the pattern). A outliers (Figure 2; Figure 4a). It includes the surface show- final view is provided by a northwest–southeast cross-sec- ings known as Happy Face, Major General and GG. In this tion from hole 64 to hole 80, which indicates a depression in

229 CURRENT RESEARCH, REPORT 99-1

Figure 4c-d. Geometric views of the North intrusion. (A) Map showing outline, drillhole locations, and section planes. (B) NDT lobe, north–south cross-section (B-B'). (C) Happy Face lobe, north–south cross-section (C-C'). (D) Happy Face lobe, cross-section near GG showing (D-D'). (E) Happy Face lobe, northeast–southwest cross-section (E-E'). (F) Happy Face lobe, northwest–southeast cross-section (F-F'). (G) Taheke Lake lobe, east–west cross-section including Northern Abitibi drilling area (G-G'). (H) Taheke Lake lobe, southwest–northeast cross-section (H-H'). Cross-sections include 200 percent vertical exaggeration for clarity. the basal contact, centred near hole 97-78 (Figure 4f). Thin in other holes. The total thickness of gabbro is probably only sequences of a dark-coloured rock containing poikilitic 50 m or so south and southeast of hole 64. olivine (in holes 58 and 80) sit between the massive leucogabbro and the mineralized sequence, and may correlate In the Happy Face lobe, the mineralized sequence is with the "black gabbro" of the Taheke Lake lobe. The latter commonly developed just below the base of the massive was also observed in hole 98-112, northwest of the Major leucogabbro unit, and there is generally no thick layered General showings (Figure 4a), and may lurk unrecognized olivine gabbro sequence. The stratigraphy, and the overall

230 A. KERR

Figure 4e-f. Geometric views of the North intrusion. (A) Map showing outline, drillhole locations, and section planes. (B) NDT lobe, north–south cross-section (B-B'). (C) Happy Face lobe, north–south cross-section (C-C'). (D) Happy Face lobe, cross-section near GG showing (D-D'). (E) Happy Face lobe, northeast–southwest cross-section (E-E'). (F) Happy Face lobe, northwest–southeast cross-section (F-F'). (G) Taheke Lake lobe, east–west cross-section including Northern Abitibi drilling area (G-G'). (H) Taheke Lake lobe, southwest–northeast cross-section (H-H'). Cross-sections include 200 percent vertical exaggeration for clarity. thicknesses of units, resemble those observed in the Worm mafic intrusion with a thickness of around 200 m, which intrusion. dips below the gneisses to the east, at about 15E (Figure 4g). However, hole 98-111, drilled less than 0.5 km west of the Taheke Lake Lobe supposed upper contact, contains no sign of a mafic intrusion of equivalent thickness. A 30 m intersection of gabbro, The Taheke Lake lobe forms the northern portion of the containing some sulphides, represents the only PLI-like North intrusion. Aside from a small (<1 km2) area, its geom- rocks observed in this hole, and has yet to be examined by etry is defined only by widely spaced drilling (Figure 4a). the author. There are several possible interpretations for An east–west section through the lobe indicates a sheet-like what happens between holes 105 and 111, as shown in

231 CURRENT RESEARCH, REPORT 99-1

Figure 4g-h. Geometric views of the North intrusion. (A) Map showing outline, drillhole locations, and section planes. (B) NDT lobe, north–south cross-section (B-B'). (C) Happy Face lobe, north–south cross-section (C-C'). (D) Happy Face lobe, cross-section near GG showing (D-D'). (E) Happy Face lobe, northeast–southwest cross-section (E-E'). (F) Happy Face lobe, northwest–southeast cross-section (F-F'). (G) Taheke Lake lobe, east–west cross-section including Northern Abitibi drilling area (G-G'). (H) Taheke Lake lobe, southwest–northeast cross-section (H-H'). Cross-sections include 200 percent vertical exaggeration for clarity.

Figure 4g. First, the hole 111 intersection could represent a increase in dip to more than 50E. Alternative explanations thinned extension (a feeder ?) to the intrusion (P. Moore and include structural disruptions, or a switch in dip direction K. Sparkes, personal communication, 1998) and second, the that brings the base of the intrusion to surface between holes intrusion could pass below hole 111, which requires a rapid 105 and 111.

232 A. KERR

The massive leucogabbro unit is mainly absent in the tory geometry of the intrusion is seen, as is the presence of Taheke Lake lobe, where there is a thick sequence of the a depression beneath Taheke Lake. A thin cap of the massive black olivine gabbro, which is present at surface. The min- leucogabbro is present locally, and the intrusion probably eralized sequence, including the usual assortment of com- continues beneath high hills of gneisses and Nain Plutonic plex rock types is persistently present at the basal contact, Suite rocks north of hole 93. However, it is not clear what and is locally very thick (e.g., 50 to 60 m true thickness in happens northwest of Taheke Lake, as there is no thick intru- hole 97-70). Deep holes such as 97-92, and 98-105, 107 and sion in hole 111 (see above discussion). The black olivine 109 indicate that the North intrusion does not include gabbro forms a discrete lower unit in this region, which is "stacked", sheet-like bodies. A long intersection of coarse- thickest in hole 108. This unit may extend all the way to hole grained plagioclase-rich rocks in hole 92 was originally con- 97-63, where thin intervals of fine-grained gabbro contain- sidered as a possible feeder system (to the massive leuco- ing poikilitic olivine sit just above, and within, the mineral- gabbro ?) but it is intruded by ferrodioritic rocks, implying ized sequence. The fine- to medium-grained, layered olivine that it is an older anorthosite. gabbro forms the central part of the intrusion.

A different perspective is provided by a Three drillholes in this area (97-93, 98-106 and 108) southwest–northeast section along the northwest edge of the provide critical geological information. In holes 93 and 106 lobe, through Taheke Lake (Figure 4h). The slightly undula- an upper fine-grained olivine gabbro is underlain by black

233 CURRENT RESEARCH, REPORT 99-1

Figure 5. A summary of the three-dimensional geometry of the North intrusion. (A) Isopachs showing the depth to the basal contact; within the outline of the body, these correspond to vertical thickness of the gabbro. Approximate extent of positive gravity anomaly is also indicated. (B) True structural contours (relative to sea level) on the basal contact of the intrusion, corresponding to a “topographic map” of the basal surface.. olivine gabbro containing interstitial to poikilitic olivine, Overall Geometry which becomes finer toward the base, and varies to a diabase-like rock. Contact relationships are fairly abrupt in hole Contour maps of the depth to the basal surface of the 93, and imply that the black gabbro is younger (Fitzpatrick intrusion and the topography of this surface provide a good et al., 1998), but the boundary is defined by a thick and het- illustration of the overall geometry of the North intrusion. erogeneous composite unit in 106. In both cases, minor sul- Thickness contours (isopachs) correlate well with regional phides are present at the contact, within the lower black gravity surveys, which show an elongate "high" along the olivine gabbro unit. Hole 108 is similar, but also contains an southeast side of Taheke Lake, extending into the centre of upper massive leucogabbro section. The fine-grained, lay- the NDT lobe (Figure 5a). Topographic contours on the ered olivine gabbro in the central part of the hole is cut by basal contact show that it is more complex, and resembles leucogabbro veins containing (relatively) chilled margins. an eroded hillside, with a central ridge running from the Similar relationships are also seen in hole 112 in the Happy Happy Face area through the centre of the Taheke Lake lobe Face lobe. Collectively, they imply that the black gabbro is (Figure 5b). The area of intense drilling actually sits on the indeed a discrete unit, which sits beneath or within the fine- top of this "ridge", which apparently plunges northward. grained unit, and has a close temporal relationship it. The Another part of this topographic high protrudes toward coarse-grained, massive, leucogabbro sits above the fine- Taheke Lake, and separates the NDT lobe from the remain- grained unit, and apparently intruded it as veins and sheets. der of the intrusion; near the northeast end of Taheke Lake,

234 A. KERR the dip of the basal surface must steepen to 25E or so. As dis- m thick, contains more abundant (but still <10 percent) sul- cussed previously, there are many uncertainties in the area phide, with a similar interstitial habit. The heaviest sulphide north and west of Taheke Lake, so no attempt has been made concentrations (up to 50 percent) are at the base, associated to portray the alternative interpretations implied by Figure with a more leucocratic gabbroic rock (Plate 6b). The sul- 4g. phides are dominated by pyrrhotite, lesser chalcopyrite and rare visible pentlandite. The host rocks contain partially This pattern resembles a north-plunging fold that has a digested, blue–grey to white gneissic fragments, some of fold axis trending at about 150E, roughly parallel to the which show dark reaction rims (Plate 6a). The mineralized regional strike of the Churchill Province gneisses. However, rocks apparently fade into a medium-grained, barren gab- there are clearly regional variations in the thickness of the bro, which is chilled against the footwall gneisses. Hole 86 intrusion, and its internal stratigraphy, which indicate that at intersected a similar sequence, including almost 90 m of least some of this geometry reflects original intrusive geom- variably mineralized rocks, showing a distinct downhole etry, with possible superposition of regional tilting and increase in the amounts of sulphides and gneissic fragments. uplift. For example, the "valley" that defines the NDT lobe The mineralized rocks contain 0.1 to 0.55% Ni, and 0.1 to may be an originally deeper section of an originally flat- 0.32% Cu (Donner Minerals, Press Release, July 1998, lying intrusion, now tilted to the northwest, and the central November 17, 1998; Fitzpatrick et al., 1998; Geological ridge could be a primary topographic high in the basal con- Survey data). Recalculation to 100 percent sulphide (see tact. The origin of the fold-like pattern is more problematic, appendix) indicates that the sulphides contain from 0.7 to but it may reflect an originally sinuous feeder system, rather 4.0 % Ni, and 0.4 to 1.5% Cu (Figure 6). Values are highest than an imposed geometry. The above discussion is specula- in the upper section, typically 2.0 to 4.0% Ni and 0.7 to tive, and there are probably several configurations that can 1.4% Cu at 100 percent sulphide, but the amount of sul- explain the known geometry of the North intrusion. phides is low. The heavier basal mineralization is relatively Calibration of regional gravity data with the drillhole information, followed by modelling of various configurations, may provide a method to evaluate various options. The problematic area between holes 105 and 111 obviously has a bearing on this problem.

MINERALIZATION

Magmatic sulphide mineralization is associated with the basal intrusive contacts of the South intrusion, Mineral Hill intrusions and the North intrusion of the PLI. The mineralized sequences of the North and Mineral Hill intrusions are similar, but they are distinct from their counterpart in the South intrusion. This section reviews mineralization and several genetically associated silicate rock types. Mineralized rocks from several early (1996) drillholes were previously described in a thesis project by Hodder (1997).

SOUTH INTRUSION MINERALIZED SEQUENCE

The South intrusion mineralized sequence is presently mostly known from drillholes 97-79, 86 and 98-100 (Figure 3), which encountered mineralization in mafic and ultramafic cumulate rocks near the base of the intrusion. If the thin zone of mineralization at the top of hole 89 is included, the total potential area of this basal mineralization is large, and it may continue north of the Pants River valley (Figure 3). Plate 6. Mineralized rocks from the South intrusion. (A) Hole 97-79 intersected two discrete mafic–ultramafic Melagabbro to peridotite containing minor interstitial sul- intervals. The uppermost (about 28 m thick), contains less phides and gneissic fragments, seen as white patches. (B) than 3 percent sulphides, which are clearly interstitial with Gabbroic unit containing magmatic sulphide mineralization respect to all silicates (Plate 6a). A lower interval, up to 50 and showing well-developed interstitial sulphide textures.

235 CURRENT RESEARCH, REPORT 99-1

Figure 6. Copper and nickel contents of sulphide mineralization in the Pants Lake intrusion. (A) South intrusion and Mineral Hill intrusions. (B) North intrusion, NDT lobe. (C) North intrusion, Happy Face lobe. (D) North intrusion, Taheke Lake lobe. All data are recalculated to 100 percent sulphides, and are mostly derived from samples with >2 wt% S. Calculation and correction methods are outlined in the Appendix. metal-poor, containing around 0.7% Ni and 0.4% Cu at 100 The South intrusion mineralized sequence appears to be a percent sulphide. Nickel–copper ratios range from slightly gravitational accumulation, where sulphide droplets and over 1.0 in lower grade areas to 2 or more in the upper inter- gneissic fragments were concentrated by the same process val. The South intrusion mineralization has higher Ni/Cu as the cumulus olivine and pyroxene. Variations in tenor ratios than elsewhere in the PLI, where Ni/Cu is mostly may represent changes in the bulk ratio of sulphide liquid to slightly above 1. silicate magma ("R-factor"; Naldrett, 1989) as the magma

236 A. KERR chamber evolved. The basal contact of the South intrusion may not contain all of the indicated components. However, represents a large and poorly explored area (Figure 3), and the two-part division into an upper heterogeneous zone and its topography is not well known. The metal contents of the a lower homogeneous zone is a fundamental characteristic. sulphides suggest that, if the sulphide liquid was able to Smith and Wilton (1998) introduced a more detailed subdi- accumulate in significant quantities in a suitable trap, the vision, which suggested specific names for rock types that area could provide an attractive exploration target. probably form part of compositional spectra. The current treatment retains the simpler terminology of Kerr (1998b), NORTH INTRUSION MINERALIZED SEQUENCE but the need for further subdivision is acknowledged.

Distribution and General Features Composite Gabbro: Petrology and Mineralogy

The North intrusion mineralized sequence is exposed in This simple term is used for a spectrum of rock types several areas along the southern boundary of the intrusion, that form the upper part of the mineralized sequence (Plate and includes the NDT, Happy Face, GG Zone and Major 7a-d). Most variants clearly contain discrete inclusion and General surface showings. The blind drilling target known matrix components. The inclusions consist of a dark grey to as the Northern Abitibi area is another area of extensive green, fine-grained to aphanitic mafic rock, which forms mineralization (Figure 2). However, sulphides are not aligned "bands" and lenses oriented subparallel to the basal restricted to these localities, as they occur in at least some contact of the intrusion, representing disk-like, ellipsoidal, quantity in almost every drillhole that penetrates the basal inclusions having a strong alignment. The coarser matrix contact of the intrusion. material is a 2 to 10 mm intergrowth of white plagioclase, clinopyroxene and olivine, associated with large clots of The surface showings are not particularly informative, intergrown sulphide. Fine-grained sulphide is locally pres- due to heavy gossan development. All are close to the basal ent within the mafic inclusions, but most is associated with contact of the intrusion. Gneisses below the contact contain the coarser matrix (Plate 7a), which accounts for its reces- local veins and patches of sulphides, but are otherwise sive weathering. The inclusions in some composite gabbros unmineralized. The lowermost rocks of the PLI are similar- are "etched" by white, fine-grained, plagioclase rims (Plate ly barren, or contain only very minor sulphide. The show- 7c). Although inclusions most commonly are ellipsoidal to ings commonly contain an upper unit that weathers to egg- rounded, some composite gabbros contain angular exam- like or disk-like masses of unweathered rock, surrounded by ples, and the textures in these are more reminiscent of intru- a friable, recessive matrix. This is underlain by more homo- sive breccias. There are also variations from rocks with geneous rocks that commonly contain disseminated sul- clear, sharp inclusion-matrix contacts to diffuse, blotchy phide. Locally, these have a "knobbly"-weathering texture rocks where fine and coarser grained domains appear almost where resistant nodules about 1 cm across are set in a reces- gradational. The proportions of inclusions and matrix vary sive, sulphide-bearing matrix. In rare fresh samples, this is and composite gabbros can be either "clast-supported" or the "leopard gabbro", consisting of clinopyroxene crystals in "matrix-supported" (Plate 7c,d). The relationships are usual- mineralized matrix. The sulphide-bearing rocks are in some ly clearer in the latter. places overlain by unmineralized fine-grained gabbro, but most commonly lie just below massive, coarse-grained, Most composite gabbros also contain rounded to amoe- leucogabbro, which is unmineralized. boid gneissic inclusions ranging from recognizable rock types (mostly granitoid gneiss) to diffuse grey or blue patch- Diamond-drill core provides the most useful informa- es having dark reaction rims (Plate 7d). These are included tion about original textures and geological relationships, and within the coarser matrix, but also locally appear to be with- much of this section is based on core samples, as reported in the fine-grained domains, forming inclusions within previously by Kerr (1998a, b). The mineralized sequences inclusions. The amount of such debris varies widely, and in show subtle variations from lobe to lobe, and are described gneiss-rich examples, it becomes difficult to discern frag- separately. However, they all contain the same rock types, ment-matrix relationships. Larger gneissic fragments local- which have a persistent stratigraphic arrangement. The ly contain vein-style sulphide mineralization (locally chal- upper part of the mineralized sequence is dominated by het- copyrite-rich), which is distinct from the interstitial style of erogeneous, breccia- like rocks collectively termed compos- matrix sulphides. Lastly, composite gabbros may also con- ite gabbro. This is underlain by more homogeneous rocks tain exotic sulphide inclusions. These are difficult to verify, containing disseminated sulphides, "leopard-textured" gab- but rounded patches of high-grade, Cu-rich sulphide bro, barren gabbro and (locally) semimassive to massive observed in composite gabbro containing pyrrhotite-rich sulphides. Idealized "stratigraphic sections" through the mineralization in hole 97-70 (Plate 7b) are difficult to mineralized sequence in the various parts of the North intru- explain by any other process. This particular example is sion are illustrated in Figure 7, but individual sequences interpreted as a transported fragment of high-grade mineral-

237 CURRENT RESEARCH, REPORT 99-1

Figure 7. Idealized stratigraphic sections through the mineralized sequence in various parts of the North intrusion. Note that not all of these components are present in a given section through the mineralized sequence. Diagram is schematic and does not convey true relative thicknesses within or between lobes. ization. Copper-rich veins in gneissic fragments likely came ophitic to poikilitic crystals (Plate 8c). Sulphides are inter- from the same type of source. A high-grade, Cu-rich clast stitial to all silicates, and commonly occur in the coarsest was also noted in the thick sulphide intersection from hole parts of the matrix (Plates 7 and 8). The textures of fine- 97-96 (K. Sparkes, personal communication, 1998). grained "inclusions" and coarser-grained "matrix" are distinct, and their mutual boundaries range from sharp (often Despite their complexity, composite gabbros have com- marked by a thin rind of fine-grained plagioclase) to fuzzy mon petrographic features. The fine-grained inclusions and gradational. Locally, coarse olivine in matrix is optical- commonly consist of a granular olivine-rich gabbro or troc- ly continuous with the olivine in inclusions with poikilitic tolite, containing tiny rounded olivines and later lath-like textures. plagioclase, with lesser late subophitic clinopyroxene (Plate 8a). These inclusions are texturally and mineralogically Composite gabbros also contain gneissic material, identical to the layered olivine gabbro unit, but finer which appears as clumps and clots of strained, metamorphic grained. In addition to "granular troctolite", some also con- plagioclase. Larger gneiss fragments are dominated by tain inclusions of a fine-grained diabase-like rock contain- strained, granular plagioclase, possibly associated with ing poikilitic clinopyroxene and olivine. More rarely, this is quartz and cordierite, surrounded by rims of dark green, the dominant or only inclusion component (Plate 8b). One granular, hercynitic spinel (Plate 8d). Locally, acicular example from the NDT lobe contained inclusions of fine- corundum is present within plagioclase (or cordierite ?) and grained norite, which has no known counterpart elsewhere is visibly replaced by dark green spinel (Plates 8d and 9a). in the PLI. The composite gabbro matrix consists of plagio- Digested foreign material is present even within samples clase, purplish clinopyroxene and grey–green olivine, with that lack visible fragments, where scattered crystals of variable amounts of biotite, magnetite and sulphides. Its tex- strained plagioclase are incorporated into the matrix igneous ture is closely similar to that of the massive leucogabbro and texture. Diffuse ring-like zones of dark green spinel repre- black olivine gabbro units, where olivine forms late sub- sent the outlines of former gneissic fragments. Locally, all

238 A. KERR

Plate 7. Examples of composite gabbro from the North intrusion. (A) Typical example of sulphide-rich material in drill core from the NDT showing, illustrating very coarse patches of matrix sulphides. (B) "Exotic" sulphide clast (?) consisting of high- grade, Cu-rich sulphide and associated silicates, hole 97-70. (C) Examples of composite gabbro dominated by fine-grained mafic inclusions in coarse gabbroic matrix, typical of the upper part of the sequence. Upper core sample shows development of discrete zones rich in gneissic debris. Note white plagioclase rims developed on many mafic inclusions. (D) Examples of composite gabbro also containing abundant digested gneissic material and matrix sulphides, typical of the lower part of the sequence. igneous phases are riddled with anhedral spinel inclusions Leopard gabbro from the GG showing essentially con- (Plate 9b). These unusual textures are regarded as the final sists of oikocrystic clinopyroxene set in a sulphide-bearing, stage in the assimilation process, where spinel restite is troctolitic, groundmass (Kerr, 1998a). Examples from other locally suspended in the magma (cf., Li et al., 1996). areas fit this basic description, with some variations (Plate 10a). The leopard gabbro is texturally similar to the unmin- Leopard Gabbro and Related Rock Types eralized, fine-grained, layered olivine gabbro, which locally shows a similar oikocrystic texture in the absence of sul- The composite gabbro is almost always underlain by phides. Sulphides are mostly confined to the groundmass as more homogeneous fine- to medium-grained gabbroic rocks a late, interstitial phase, but larger patches also occur, which that contain disseminated sulphide mineralization. The contrast texturally with the groundmass texture (Plate 10b). "leopard gabbro" is the best-known example and is textural- Some were earlier suggested to be exotic sulphide fragments ly equivalent to the "leopard troctolite" at Voisey's Bay (Kerr, 1998a), but they could also represent coalesced sul- (Naldrett et al., 1996). The leopard gabbro is actually one phide droplets. However, the bimodal size distribution is variant of a range of gabbroic rocks containing disseminat- striking, and at least some larger sulphide clots are associated, interstitial mineralization, and is gradational with these ed with coarse mafic silicates, including clinopyroxenes closely related rocks. with orthopyroxene cores, unknown elsewhere in the PLI.

239 CURRENT RESEARCH, REPORT 99-1

Plate 8. Petrography of composite gabbro unit. (A) Heterogeneous sample showing three main components; from left to right, these are (i) fine-grained, granular, troctolitic gabbro inclusion, (ii) coarse-grained gabbroic matrix with sulphides and (iii) a metasomatized gneiss fragment containing abundant spinel. (B) Similar, heterogeneous rock, but here containing fine- grained diabase-like inclusions with poikilitic clinopyroxene. Rim of a gneissic fragment at lower right. (C) Coarse-grained matrix material showing interstitial olivine, clinopyroxene and sulphides. (D) Gneissic fragment, showing acicular corundum, reacting to form green spinel. Pseudohexagonal pattern may indicate that parent mineral was largely cordierite, rather than plagioclase. Ol – olivine, Pl – plagioclase, Cpx – clinopyroxene, S – sulphide, Sp – spinel, Co – corundum, Gf – gneiss fragment, dbs = diabase inclusion. All photos are in PPL (plane-polarized light).

Plate 9. Reaction textures in gneissic inclusions. (A) Coarse green spinel and associated corundum in the central region of a gneissic fragment. (B) Granular spinel inclusions, seen as tiny dark patches, contained within plagioclase and olivine crystals in composite gabbro matrix. These textures suggest complete disaggregation of restite from assimilated gneisses. Ol – olivine, Pl – plagioclase, Cpx – clinopyroxene, Sp – spinel, Co – corundum. All photos are in PPL (plane-polarized light).

240 A. KERR

Plate 10. Examples of more homogeneous mafic rocks from the mineralized sequence of the North intrusion. (A) From top to bottom of photo, a downhole progression from essentially barren gabbro to leopard gabbro, and to semimassive sulphide containing clinopyroxene crystals, hole 98-103. This indicates gravitational sulphide accumulation. (B) Contrasting habits of sulphide mineralization, hole 98-112; note bimodal pattern of very fine-grained interstitial sulphide, and larger, rounded sulphide patches. (C) Drill-core examples of mineralized poikilitic diabase, holes 97-67 and 97-70. (D) Petrography of mineralized poikilitic diabase; note that olivine, clinopyroxene and sulphide all form poikilitic crystals. Ol – olivine, Pl – plagioclase, Cpx – clinopyroxene, S – sulphide. Photo in PPL (plane- polarized light).

Most leopard gabbros contain gneissic fragments, which interlayered with typical leopard gabbro, which seems to resemble those in the overlying composite gabbro, and have have formed by gravitational accumulation of sulphides reaction rims of granular hercynitic spinel. These ring-like (Plate 10a). Hole 98-103 from the Northern Abitibi area patterns resemble features described in texturally similar contains at least two "cyclic" units that progress downward rocks at Voisey's Bay (Li et al., 1996). Clots of metamorphic from virtually barren gabbro, into disseminated sulphide, plagioclase and anhedral spinel inclusions in all igneous incipient leopard texture, and true leopard gabbro. One such minerals provide local evidence for near-total digestion of unit eventually grades down into semimassive sulphides gneissic inclusions. However, the proportion of xenolithic containing clinopyroxene oikocrysts, that represents an and digested material is much less than in the overlying "ultimate" leopard-texture. These features, and the general composite sequence, except close to their mutual boundary, homogeneity of the rocks, suggest that the lower part of the where it may be high. mineralized sequence represents a relatively "quiet" magmatic environment compared to the upper part. The leopard gabbro is gradational with other fine- grained gabbros containing dispersed sulphide. Some also The more homogeneous mineralized rocks also locally contain clinopyroxene oikocrysts, but lack sufficient sul- include recognizable fragments of other rock types that were phide to delineate them, but others contain only interstitial previously mineralized. Hole 96-43 at the GG zone contains clinopyroxene, and are texturally identical to typical fine- xenoliths of coarse-grained clinopyroxene monzonite with grained olivine gabbro. In several drillholes, these rocks are interstitial granophyre, which contains invasive magmatic

241 CURRENT RESEARCH, REPORT 99-1 sulphide veins, clearly truncated by the surrounding fine- against gabbro that may be a two-liquid boundary, as round grained gabbro. The xenoliths do not correspond to any blobs of sulphide are present in the overlying gabbro and known PLI rock, and are interpreted as older vice-versa (Plate 11a). High-grade sulphide zones, such as Mesoproterozoic igneous rocks that hosted footwall-style the famous hole 75 intersection (11.7% Ni and 9.7% Cu) are mineralization at some other location prior to becoming mostly known from the footwall region, and small zones of entrained in the host magma. this type are particularly common in the Northern Abitibi area (Plate 11c). Hole 98-131, however, contained massive Mineralized Poikilitic Olivine Diabase sulphides with 4.5% Ni and 2.6% Cu at the basal contact of the gabbro. The lower part of the mineralized sequence locally also includes another fine-grained, mineralized rock type which Massive sulphides display considerable textural varia- is distinct from leopard gabbro (Plate 10c). This has a strik- tion. Some are coarse-grained, "clean" sulphides consisting ing diabase-like texture, in which plagioclase laths are of pyrrhotite, chalcopyrite and pentlandite, which locally enclosed by a network of larger, poikilitic olivine and show "loop textures" (i.e., rings of pentlandite around clinopyroxene crystals. The plagioclase is typically dark in pyrrhotite crystals) akin to those described from the Eastern drill core and contains oxide inclusions. Sulphide mineral- Deeps deposit at Voisey's Bay (Naldrett et al., 1996). High- ization is also interstitial and late, and locally forms grade footwall sulphide veins have a coarse crystalline tex- poikilitic crystals that include plagioclase (Plate 10d). This ture reminiscent of Voisey's Bay material (Plate 11c), which may have a textural equivalent in fine-grained, diabase-like is also seen in the hole 131 intersection. Lower grade, inclusions seen in some composite gabbros, but it is also pyrrhotite-dominated massive sulphides tend to be finer texturally akin to the composite gabbro matrix. It contains grained and "dirty" having variable grain sizes, texture, scattered digested gneissic inclusions with the usual corun- abundant silicate inclusions, quartz grains and graphitic dum-spinel reaction products. In drill core, well-preserved patches (Plate 11b). These have either assimilated country examples having lesser amounts of sulphide resemble fine- rocks through thermal erosion, or are actually partially grained versions of the black olivine gabbro unit. It is very derived by melting and mobilization of primary sulphides difficult to distinguish from one fine-grained, dark mafic from the metasedimentary gneisses. The sulphide mineral rock from another, and the distribution of this rock type assemblages are also consistent with generation from such a within the mineralized sequences remains poorly known. source (A.J. Naldrett, personal communication, 1998; and unpublished consultant reports). Barren Olivine Gabbro Footwall Gneisses The mineralized rocks described above commonly pass down gradationally into a fine-grained, sulphide-free gab- It is commonly difficult to position the basal contact of bro. Most examples are texturally identical to the fine- the North intrusion precisely, especially if massive sulphides grained, layered olivine gabbro. Chilled versions adjacent to are present. The underlying gneisses have not yet been the footwall contact show diabase-like textures and ophitic examined in any detail. The most common rock type is a clinopyroxene, which reflect rapid cooling. coarse-grained garnetiferous orthogneiss, which underwent contact metamorphism and reaction for 25 to 50 m below Massive Sulphide the basal contact. Garnet has been transformed to grey cordierite and/or orthopyroxene pseudomorphs, but is pro- Massive sulphide horizons, where present, are located gressively better preserved at depth. A peculiar texture, in close to the footwall gneiss contact, and at least some are which grain boundaries are "etched" by fine-grained biotite probably located below its original position, as they contain and/or amphibole aggregates, is also spatially associated gneissic inclusions, xenocrystic quartz, and graphite. with the contact region. Invasive, vein-style sulphide miner- Relationships between massive sulphides and spatially asso- alization is also common in the immediate footwall region. ciated mineralized gabbros are ambiguous. Thin massive Kerr (1998a) also reported the presence of hercynitic spinel sulphide veins cut leopard gabbro and associated rock types, and corundum, which are characteristic of the more digest- and massive sulphides at the footwall contact are locally iso- ed inclusions seen in the mineralized sequence. The footwall lated from the overlying mineralized sequence by chilled, rocks likely preserve the initial stages of reactions that even- barren gabbro. Vein-style massive sulphides also commonly tually led to the digestion and assimilation of gneissic coun- cut the gneisses below the footwall contact. These relation- try rocks, and they are an important avenue for future ships imply that sulphide liquids remained mobile after research in the area. They also provide a possible method of solidification of the overlying rocks, and have migrated evaluating the emplacement depth of the PLI through study along the footwall contact region. However, hole 97-67 con- of contact metamorphic assemblages. tains a massive sulphide zone with an unusual upper contact

242 A. KERR

Plate 11. Examples of massive sulphide intersections from the Taheke Lake lobe of the North intrusion. (A) contact of fine- grained gabbro and massive sulphide zone, hole 97-67. Note the rounded inclusions of sulphide in gabbro, and gabbro in sulphide, which may indicate a two-liquid interface. (B) Heterogeneous massive sulphides, hole 98-101, littered with gneissic debris, and showing contrasts in grain size and texture. Footwall paragneiss at lower right, gabbro at upper left. (C) High- grade massive sulphides (up to 11.8% Ni, 9.7% Cu) from footwall vein in hole 97-75, showing coarse crystalline texture. (D) Sulphide veins invading chilled barren gabbro and footwall gneisses, adjacent to the basal contact of the gabbro, hole 98- 109.

NDT Lobe Mineralized Sequence resemble composite gabbro, albeit without sulphides. Locally, the coarse-grained material appears to be included The mineralized sequence in the NDT lobe is situated in finer grained material, which is the reverse of the normal below a thick sequence of fine-grained, layered olivine gab- relationship. The main part of the mineralized sequence is bro, and is well removed from the base of the massive dominated by composite gabbro, but this has an unusual coarse-grained leucogabbro unit (Figure 4). Hole 96-04, "stoped" texture, where inclusions are subangular, but drilled west of the showings, encountered about 30 m of remain strongly aligned. This is underlain by only 1 to 2 m spectacular composite gabbro, sitting beneath a medium- of fine-grained, sulphide-bearing gabbro. Other holes in the grained olivine gabbro or troctolite containing cumulus NDT lobe display a similar pattern of a relatively thick com- olivine. Gneissic fragments and derived restite become more posite sequence, underlain by a thin zone of homogeneous abundant toward the base, and fine-grained diabase inclu- fine-grained gabbro. In general, the amount of sulphide in sions accompany the dominant granular troctolite. The com- the mineralized sequence is low (2 to 7 percent), and mas- posite sequence is underlain by only a few metres of homo- sive sulphides are essentially absent. The NDT mineralized geneous fine-grained gabbro containing disseminated sul- sequence has low absolute Ni and Cu contents (< 0.30% phide. In hole 77, almost 300 m of fine-grained, layered combined) reflecting the low sulphide abundance. Nickel olivine gabbro sits between the base of the massive leuco- contents at 100 percent sulphide are consistent at 1.5 to gabbro and the top of mineralized rocks (Figure 4). The low- 2.0% Ni, commonly with equivalent or marginally lower Cu ermost 20 m of the fine-grained unit is a complex mixed contents (Figure 6). zone, including heterogeneous inclusion-rich sections that

243 CURRENT RESEARCH, REPORT 99-1

Happy Face Lobe Mineralized Sequence Taheke Lake Lobe Mineralized Sequences

In the Happy Face lobe, the North intrusion is a thin In the Taheke Lake lobe, the mineralized sequence sits sheet dominated by massive leucogabbro, and the mineral- at or just below the base of the black olivine gabbro unit, ized sequence commonly sits just below the base of this which occurs at surface in the east, but sits beneath fine- upper unit, with no significant intervening thickness of fine- grained, layered olivine gabbro in the west (Figure 4). There grained gabbro (Figure 4). The mineralized sequence con- are marked variations in the thickness of the mineralized tains the standard pattern of an upper composite gabbro sec- sequence in this area, and it is thickest in hole 97-70, where tion underlain by more homogeneous rocks. Gneissic frag- nearby drillholes indicate a progressive thickening to the ments are particularly abundant at the base of the composite northeast. gabbro section. The lower homogeneous zone is thick (20 m true thickness at the GG showing) and locally shows well- The mineralized sequence shows the familiar division developed leopard texture. A weakly mineralized to barren into an upper section dominated by composite gabbro, lower zone is also well developed at the GG showing. Other underlain by more homogeneous gabbros containing dis- examples of the mineralized sequence from the Major seminated sulphide, which are locally leopard textured General and Happy Face showings are similar, but the lower (Figure 7). In general, the composite section is thicker than zone of homogeneous rocks is thinner. East of the showing, the more homogeneous zone, with almost 50 m of compos- it is locally absent, and composite gabbro sits directly on ite gabbro in hole 70. In hole 97-67, fine-grained gabbro gneisses. Composite gabbros at the Major General showing containing granular, cumulus olivine sits between the base (e.g., hole 96-20) include both granular troctolite and of the black olivine gabbro and the top of the composite gab- poikilitic diabase inclusions. Two holes that contain the bro, but the underlying homogeneous section includes min- black olivine gabbro unit are of particular interest. In hole eralized poikilitic olivine diabase that is texturally similar to 112, medium-grained black olivine gabbro seems to pass black olivine gabbro and composite gabbro matrix. Some directly into composite gabbro, underlain by 5 m of homo- other holes in this area show a similar pattern, but elsewhere geneous gabbro with disseminated sulphide. A similar the black gabbro passes directly into composite gabbro, and sequence is seen in hole 80, where the sulphides appear to in one example it passes straight down into poikilitic olivine be pentlandite-rich compared to most other areas. In both diabase with interstitial sulphide. The composite gabbro sec- holes, the black olivine gabbro appears to sit beneath typical tion shows a marked downhole increase in gneissic debris, fine-grained, layered olivine gabbro, and the mineralized and possibly also in sulphide content. The more homoge- sequence is isolated from the upper massive leucogabbro. neous gabbros beneath the composite section show gravitational sulphide accumulation in the development of leopard Absolute grades in the Happy Face lobe are variable, gabbro (e.g., hole 97-103, see above discussion, also Plate with the best results around 0.6% Ni, 0.45% Cu and 0.03 to 10a). The transition from composite to homogeneous 0.14% Co. The higher absolute grades represent rocks hav- sequences locally shows "interbedding" of the two rock ing a wide range of sulphide contents, from around 20 to 75 types, suggesting that there must be lateral as well as verti- percent or more. When recalculated to 100 percent sulphide cal facies changes. Barren gabbro, locally chilled, is sporad- (see appendix), the samples resolve into two overlapping ically present at the base of the sequence. Massive sulphides groups (Figure 6), and the highest metal values come from generally occur just above the footwall contact or within composite gabbro samples in the upper part of the sequence, gneisses and, in some cases, invade chilled basal gabbro and where sulphides locally contain up to 3.9% Ni and 3.1% Cu overlying leopard gabbro. (in hole 97-80). The Leopard gabbro and similar rocks in the lower part of the mineralized sequence show 0.6 to 1.8% Ni The Taheke Lake lobe shows the greatest variations in and 0.5 to 1.4% Cu at 100 percent sulphide, with the best absolute metal grades, which range up to 11.8% Ni, 9.7% values at the GG zone itself. The contrast in tenor through Cu and 0.43% Co in the famous hole 75 intersection. The the mineralized sequence appears to be independent of sul- highest absolute grades that can be spatially linked to the phide content, i.e., samples with similar sulphide contents mineralized sequence are 4.5% Ni and 2.6% Cu in a thin show contrasting tenors. A general tendency for samples zone from hole 98-131, which contains only about 60 per- with small amounts of disseminated sulphide to show high- cent sulphides, indicating sulphide metal contents of 8.7% er metal contents than samples with higher sulphide con- Ni and 5% Cu (Donner Minerals, Press Release, November tents has been noted in other studies (e.g., Barnes et al., 17, 1998). Most of the disseminated mineralization contains 1996) but this does not seem to be the only factor at work 1.5 to 2.0% Ni and 1.2 to 1.8% Cu at 100 percent sulphides here. (Figure 6). The data from the Taheke Lake lobe also illus-

244 A. KERR trate grade contrasts within the mineralized sequence. sulphide mineralization from the PLI North intrusion shows Samples from the more homogeneous lower section have consistent Ni/Cu ratios of slightly greater than 1, and high lower Ni and Cu contents at 100 percent sulphide, but this correlation of Ni and Cu (Figure 8 left). Some high-grade pattern is far from universal. Anomalously high metal con- sulphide intersections (notably hole 75) also lie on this tents, from 2.7 to 6% Ni and 3.3 to 8.5% Cu at 100 percent trend, but others are anomalously rich in either Ni or Cu, sulphide, occur in the lower part of the thick mineralized suggesting that a fractionation process was involved in their sequence in hole 70. Although high-grade sulphide frag- generation. The South intrusion mineralized sequence ments were observed in one part of this interval, their fre- shows distinctly higher Ni/Cu, commonly >2.0, aside from quency does not seem high enough to fully explain this material at its very base (Figure 6). Mineralization from result, or the tendency for low Ni/Cu ratios in part of this Voisey's Bay itself mostly shows Ni/Cu ratios >2.0, in the hole. same range as the South intrusion (Figure 8 right). However, massive sulphides at the Voisey's Bay deposit commonly MINERAL HILL MINERALIZED SEQUENCES contain 4% Ni or more, which is higher than most calculated sulphide metal contents from PLI mineralization (Figure The Mineral Hill area has seen only limited exploration 6). The PLI mineralization also shows low Ni/Co ratios since 1996, and its mineralized sequence is not well docu- (average 8 to 9) and, for a given Ni content, the sulphides mented. Most of the drillholes examined to date (including contain significantly more Co than at Voisey's Bay. 96-36, described previously by Kerr, 1998a) are dominated Nickel/cobalt ratios for the Reid Brook Zone are closest to by relatively homogeneous, fine- to medium-grained gabbro those from PLI mineralization. The South intrusion mineral- containing evenly dispersed magmatic sulphide. These min- ized sequence also maintains a fairly low Ni/Co ratio. eralized rocks resemble the fine-grained gabbro in the north Comparisons of Co data at 100 percent sulphides have not intrusion, and contain euhedral to granular cumulus olivine. been attempted, as these are sensitive to assumptions about Clinopyroxene oikocrysts, and typical leopard texture, are correction factors for silicate material; however, the empiri- developed sporadically. The mineralized gabbros also con- cal data from massive sulphide zones in the PLI implies val- tain scattered gneissic fragments, which exhibit the green ues in the 0.2 to 0.3 percent range. Higher Co values can spinel rims seen in the north. Locally, spinel forms inclu- result from enrichment of Co in the source magma, or from sions in all igneous phases, and strained metamorphic pla- lower R-factors during sulphide segregation because the sul- gioclase masquerades as an igneous phase. However, phide/magma partition coefficients for Co are less than despite this evidence for contamination, none of these rocks those for Ni and Cu (Naldrett, 1989). show the extreme complexity of composite gabbros seen in the north. However, holes 96-45, 48 and 49 are described as containing heterogeneous rocks with fine- and coarse- SUMMARY AND DISCUSSION grained domains, containing sulphides, that sit above the main mineralized zone. These zones are relatively thin, and The final section of the report is intended to highlight were not sampled, but they are probably similar to the com- some of the points that emerge from the work conducted to posite gabbro sequence present in the north. date, and also to provide a general evaluation of some of the parallels and divergences between the study area and Despite their fairly high sulphide contents, locally > 20 Voisey's Bay itself. These are of necessity preliminary, as percent, mineralized samples from these zones have low detailed accounts of the Voisey's Bay area are only now grades, typically less than 0.3% combined Ni and Cu. entering the publication stage. The discussion here is not Recalculation of disseminated mineralization indicates low intended to provide a full and exhaustive treatment, but to tenors of 0.2 to 0.8% Ni and 0.1 to 0.6% Cu (Figure 6), but provide a starting point for more detailed studies. the amount of data is small. Minor sulphide intersections in the upper sections of holes 79 and 94 (South intrusion; see PETROLOGY AND COMPOSITION OF THE PANTS Figure 3) show similarly low values, as does a sulphide zone LAKE INTRUSION in one of the "stacked" sheets in hole 110. Overall, these grades resemble those obtained from the lower part of the The PLI consists dominantly of olivine gabbro, mineralized sequence in some parts of the North intrusion. although it does include some troctolites. However, these The tenor of sulphides in any composite sequence at Mineral are uncommon, and more homogeneous examples are prob- Hill is presently unknown. ably cumulate rocks related to the layered olivine gabbro. GEOCHEMICAL CHARACTERISTICS OF Also, many of the fine-grained mafic inclusions observed in SULPHIDE MINERALIZATION composite gabbro consist of granular troctolite and olivine- rich gabbro. The most primitive and magnesian member of The mineralization in the PLI shows a marked geo- the PLI is the fine-grained, layered, olivine gabbro. The chemical coherency. Both disseminated and more massive massive leucogabbro and black olivine gabbro units are gen-

245 CURRENT RESEARCH, REPORT 99-1

Figure 8. Nickel–copper–cobalt relationships in the PLI mineralization, compared to the Voisey's Bay area. (Left) Cu–Ni plot. (Right) Ni/Co–Ni/Cu plot. Both diagrams use absolute Ni, Cu and Co contents, not the 100 percent sulphide values employed in Figure 6. Fields for Voisey's Bay data are derived from assay results from Diamond Fields Resources and Voisey's Bay Nickel Company press releases, from 1995 to 1998, and may not be fully representative of their characteristics, especially for the Reid Brook zone. erally more pyroxene-rich, and contain olivine of late, inter- REGIONAL SETTING OF THE PLI stitial to poikilitic type, which contrasts with the granular olivine of the fine-grained layered unit. The PLI was emplaced in a static environment into dominantly metasedimentary gneisses of the Churchill All units of the PLI represent relatively primitive, Province, which locally carry significant primary sulphide unevolved mafic rocks in comparison to the leucocratic and graphite. The lithological assemblage is similar to the norite and anorthosite that predominates amongst Tasiuyak gneiss farther north (Figure 1), but is less strongly Mesoproterozoic suites in Labrador. Its general features mylonitized. The Tasiuyak gneiss surrounds some of the invite comparison with the "troctolitic" rocks of the Voisey's Voisey's Bay deposits, and has been implicated as a source Bay district, and it is similar to the northern part of the Reid of sulphur in the ores and of gneissic fragments seen in host Brook intrusion (now known as the Mushua intrusion), troctolites (e.g., Naldrett et al., 1996). Stable isotope data which consists of an early layered unit and a later, massive are ambiguous, but certainly permit a significant contribu- leucocratic unit, dated at 1317 and 1313 Ma respectively tion of sulphur from the Tasiuyak gneiss (Ripley et al., 1997, (Amelin et al., 1997, 1998). Mineralization at Voisey's Bay 1998). The presence of similar sulphide-bearing gneisses in is linked to a slightly older 1333 Ma unit (the Voisey's Bay the environment of the PLI implies that a crustal source of intrusion), which consists mostly of more massive trocto- sulphur (if needed) is readily available. A contamination sig- lites. Most of the mafic rocks in the Voisey's Bay area con- nature attributed to the effects of the Tasiuyak gneiss is also tain variable amounts of clinopyroxene, and many are one of the principal isotopic characteristics of the Voisey's olivine gabbro if classified strictly. Distinctions between Bay intrusion, compared to the Mushua intrusion (Amelin et "troctolite" and "olivine gabbro" intrusions are probably not al., 1998), and may therefore be an important factor in ore significant and, at least in terms of major-element geochem- genesis. Although equivalent data are not yet available for istry, the PLI certainly resembles some of the Voisey's Bay the PLI, there is certainly field and petrographic evidence area troctolitic rocks (Emslie, 1996; A. Kerr, B. Ryan, for such contamination. unpublished data). Geochronological data, as referenced earlier, indicate that the PLI was emplaced during the same The mafic magmas that host the Voisey's Bay deposits time interval. are considered to have ascended through structures related

246 A. KERR to the Nain Province–Churchill Province boundary, which is identification of feeder systems is likely to be important, depicted as passing through the deposit area (Ryan et al., because all significant mineralization at Voisey's Bay 1995; Naldrett et al., 1996). In the area of the PLI, the equiv- appears to be within, or linked to, discrete feeder structures alent boundary lies 10 to 20 km to the east. However, it is (Naldrett et al., 1996; Evans-Lamswood et al., 1998). far from clear that the true Nain Province–Churchill Province boundary lies at Voisey's Bay, as some of the "Nain CONTAMINATION OF THE PLI BY gneisses" in this area could be Archean basement of the METASEDIMENTARY GNEISSES Churchill Province (Ryan, 1996, personal communication, 1998). Prevailing models for the genesis of magmatic sul- Most of the PLI consists of homogeneous gabbro, phide deposits, which emphasize the importance of an exter- which generally lacks inclusions or xenoliths of country nal sulphur source, imply that it is more important to be on rock. Preliminary analysis of geochemical data (A. Kerr, the correct side of the Nain–Churchill boundary than right unpublished data) does not show compelling evidence for on top of it. extensive crustal contamination in these "normal" rocks. However, the rocks of the mineralized sequences in all areas GEOMETRY, SIZE AND EROSION LEVEL OF THE must be contaminated because numerous, variably digested PLI gneissic fragments, including recognizable metasedimentary rocks, provide evidence for incorporation of this materi- Component bodies of the PLI are flat-lying or inclined al. Complex reactions, which involve the generation of sheet-like intrusions, but their dimensions vary consider- corundum and hercynitic spinel, indicate metasomatic trans- ably. The South intrusion is a slab-like, chamber over 500 m formation of inclusions, and contamination of the host thick, and the North intrusion is also locally more than 400 magma. Strained metamorphic plagioclase masquerading as m thick. Both represent significant volumes of mafic an igneous phase, and dispersed spinel inclusions in all magma, and thus significant potential sources for Ni and Cu. igneous phases, indicates that even some relatively homoge- The geometry of the North intrusion is complex, and far neous members of the mineralized sequence must be con- from fully resolved, but it is clear that the intrusion includes taminated. Contamination is known to be widespread in the significant subsurface extensions. Other parts of the PLI mineralized rocks of the Voisey's Bay area, and the "basal represent fairly small bodies, with true thicknesses less than breccia sequence" (to which the composite gabbros are pos- 200 m, and locally less than 100 m, but they must be con- sibly equivalent, see below) is in part defined by digested nected in some way to the larger bodies, as they contain gneissic inclusions. The corundum and spinel-generating essentially identical rock types and mineralization. reactions observed in composite gabbro resemble those noted in contaminated rocks at Voisey's Bay (Naldrett et al., The combination of an original sheet-like geometry and 1996; Li et al., 1996). Contamination by metasedimentary erosion means that there are large areas, particularly in the material is also the main distinction between the extensive- north, where basal contacts lie at or close to the erosion sur- ly mineralized Voisey's Bay troctolites and the lesser known face. This resembles Voisey's Bay, where the basal contacts Mushua troctolites (Amelin et al., 1998). of the Mushua and Voisey's Bay intrusions are similarly exposed, but the total surface and shallow subsurface extent NATURE AND STRATIGRAPHY OF MINERALIZED of basal contact regions is much more extensive in the PLI. SEQUENCES This is an important indicator of potential, because most magmatic Ni–Cu deposits are associated with the basal Comparisons between the mineralized sequences of the regions of genetically associated intrusions (Naldrett, 1989). PLI and those of the Voisey's Bay area are hampered by a The original dimensions of PLI intrusions are fairly well lack of published information concerning the latter. constrained, because their roof zones are exposed and pene- However, most of the sulphide mineralization at Voisey's trated by drilling. In the case of Voisey's Bay, the original Bay is hosted by, or associated with, discrete feeder conduit dimensions of the intrusion(s) are totally unknown, and systems, such as the dyke-like body that hosts the Ovoid, could conceivably have been much larger, perhaps on the Discovery Hill and Reid Brook zones (Naldrett et al., 1996, scale of the Kiglapait intrusion (Figure 1). A second differ- 1998; Evans-Lamswood et al., 1998). Although there is per- ence is that steeply inclined to vertical feeder conduit sys- sistent disseminated sulphide in the basal region of the tems have not yet been identified in the PLI. Conduit sys- Eastern Deeps troctolite body, the most extensive mineral- tems of this type, however, could occur in many areas ization is at the entry point of a feeder sheet (Naldrett et al., beneath the North and South intrusions, and would be 1996; Evans-Lamswood et al., 1998). Drilling to date has extremely difficult to detect by vertical drilling. As dis- not clearly identified comparable conduit systems in the cussed above, there is a possible candidate for a low-angle PLI. The Worm intrusion, superficially the most obvious feeder system northwest of Taheke Lake (hole 98-111; P. candidate, has a stratigraphy that is closely similar to con- Moore and K. Sparkes, personal communication, 1998). The nected parts of the North intrusion, and may simply be a dip-

247 CURRENT RESEARCH, REPORT 99-1 ping extension of it. The thin gabbro intersection northeast through) partly consolidated and/or partly solidified mafic of Taheke Lake may also be a candidate, but its relationship magma. The inclusion–matrix textures observed in drill core to the main part of the gabbro has yet to be established. resemble those observed in outcrops where mixing is inferred to involve the "freezing" of one magma against The distribution and overall pattern of mineralized another (e.g., Vernon, 1984; Weibe, 1987), which common- sequences in the PLI may differ from the Voisey's Bay ly results in contradictory intrusive relationships. The "spec- region. In the South intrusion, a basal mineralized sequence tral variation" amongst composite gabbros (Plate 7) reflects is associated with mafic cumulate rocks, and may have a variety of controls, but the most important are the compo- developed through gravitational segregation of sulphide. In sitional and thermal state of end-members, and the degree of the North intrusion, there is a remarkably persistent miner- mixing, which are interdependent. Although there are exam- alized sequence at the base of the body. Although much of ples where inclusions appear to have been derived from the drilling that defines it was targeted on geophysical largely solid rocks (e.g., hole 97-77), most inclusions were (notably EM) responses, the consistency of its stratigraphy likely at least semi-liquid, and froze when entrained by the and common chemical patterns implies that there is some matrix magma. Blotchy, "curdled" rocks where matrix– basal mineralization everywhere. The internal stratigraphy inclusion relationships are indistinct represent advanced consists of an upper composite gabbro sequence, rich in stages of mixing and hybridization, whereas the process has gneissic debris, underlain by more homogeneous gabbroic been more commonly been arrested. Gravitational and/or rocks containing disseminated sulphides, which are locally fluid dynamic effects also influence the characteristics of isolated from the footwall contact by barren gabbro (Figure these rocks, in particular the abundance of gneissic frag- 7). It is difficult to escape the conclusion that mineralization ments and sulphides. The most xenolith-rich and sulphide- occurred as part of a single broad event, or closely spaced, rich variants are commonly in the lower part of the compos- correlative events, throughout the North intrusion. ite sequence, implying gravitational settling. Composite gabbro almost invariably shows a strong alignment of inclu- The stratigraphic arrangement of the different parts of sions and locally shows apparent clast imbrication. The fab- the mineralized sequence is also distinct. At Voisey's Bay, ric in these rocks is commonly subparallel to the base of the the basal breccia sequence (similar to composite gabbro) is intrusion, and it is suggested to result from lateral flow. the lower unit in the feeder sheet, and more homogeneous sulphide-bearing rocks (including leopard troctolite) sit The fine-grained domains could be derived locally from above it (e.g., Naldrett et al., 1996). More recent discussion the layered olivine gabbro unit, (which they closely resem- suggests that there may be more complex vertical- and lat- ble) or from other mafic magmas in the intrusion pathway. eral-facies variations in the feeder system, related to Given that some composite gabbros contain different types changes in fluid-dynamic environment (Evans-Lamswood of mafic inclusions, it is more likely that they come from et al., 1998). However, the PLI mineralized sequence has several different sources. The affinity of the coarser, sul- the opposite stratigraphy, with composite gabbro almost phide-bearing matrix is harder to establish but, in textural everywhere sitting above leopard gabbro and related rocks, and mineralogical terms, it resembles both the massive and it does not show signs of strong regional lateral-facies leucogabbro and black olivine gabbro units, as it contains variations, although they are certainly present at a local their distinctive late, interstitial to poikilitic olivine crystals. level, as indicated by alternation of composite gabbro and The contrast in olivine habit (granular and early in inclu- leopard gabbro in some drillholes. sions, poikilitic and late in matrix) suggests that the two components represent different magmas, albeit of similar NATURE AND SIGNIFICANCE OF COMPOSITE general composition. Plagioclase rims developed around GABBRO inclusions in some examples may record initial crystallization of the matrix magma, which must have had plagioclase These rocks are the common thread in the North intru- on its liquidus before olivine and pyroxene, on textural evi- sion mineralization, and must be fundamental in its devel- dence. opment. Composite gabbro appears to have many affinities to the "basal breccia sequence" at the Voisey's Bay deposit The gneissic fragments in composite gabbro must (Naldrett et al., 1996), but the two rock types may not be include some local material, but their digestion and reaction direct equivalents. Detailed comparisons are presently ham- implies that most have come from elsewhere, and had pro- pered by a lack of published petrographic information from tracted residence times in the magma. As discussed above, the Voisey's Bay area. these xenoliths have experienced reactions similar to those documented at Voisey's Bay (Naldrett et al., 1996; Li et al., The preferred interpretation for composite gabbro 1996), and they provide direct indications of extensive and sequences involves mixing processes where a sulphide- and pervasive contamination. fragment-rich magmatic pulse is emplaced into (or passes

248 A. KERR

The identity of the matrix material in the composite rocks (0.4 to 1%). Sulphide introduced into a preexisting gabbro is of fundamental importance, because the sulphide magma should not suffer significant loss of Ni, and would mineralization is always associated with it. In the Happy more likely increase its Ni content, because its R-factor is Face lobe and the Worm intrusion, the mineralized sequence raised, and some of the sulphur would likely dissolve in an sits just below the massive leucogabbro unit, which led Kerr undersaturated magma, leading to higher Ni concentrations (1998b) to suggest a link between the two. However, else- in that which remains. An alternative explanation, also sug- where there is no such spatial association, and composite gested by Kerr (1998b) is that the lower mineralized gabbro sits below fine-grained, layered olivine gabbro or sequence represents a separate (probably earlier) pulse of black olivine gabbro. The black olivine gabbro locally pass- sulphide-bearing magma. This interpretation still permits es straight down into composite gabbro, and also into a finer some interaction between mineralized pulses, and fits with grained sulphide-bearing rock having a similar poikilitic the local presence of disseminated mineralization in com- olivine texture. Field work and drill core examination in posite gabbro inclusions. It also allows the presence of 1998 suggests that the black olivine gabbro may be more "exotic" sulphide fragments in the composite gabbro, widespread than originally thought, and its finer grained although the problems in verifying these are undiminished. variants are very difficult to identify. Finally, it may be significant that composite gabbro-like rocks are locally devel- It is too early to speculate, in detail, about the relative oped outside the mineralized sequence, notably at the upper timing of mineralization, or to propose any type of model. contact of the black olivine gabbro in the Taheke Lake lobe. However, it seems likely that the mineralized sequence(s) Although these generally lack gneissic fragments, they do represent fairly early influxes of magma, as they are situat- contain minor sulphides. These "upper" composite intervals ed near the base of the intrusions. However, the normal laws are interpreted to record mixing between black gabbro par- of stratigraphic superposition do not apply in magmatic ent magma and fine-grained olivine gabbro, although the environments, and there was undoubtedly some resident order of emplacement is not entirely clear. If the black magma in the chamber at the time of their arrival. olivine gabbro is equivalent to the composite gabbro matrix, understanding its distribution in the North intrusion is cru- POTENTIAL FOR MASSIVE SULPHIDE cial in further exploration work. MINERALIZATION The most obvious contrast between the Voisey's Bay NATURE AND SIGNIFICANCE OF THE LOWER and PLI areas is that exploration activity in the latter has not MINERALIZED SEQUENCE yet resulted in the discovery of any large, potentially economic, concentrations of massive sulphides such as the In the relatively static environment represented by the Ovoid or Eastern Deeps deposits. However, the Voisey's lower part of the mineralized sequence, sulphides were able Bay deposits occur in a small and intensely explored area, to settle gravitationally, and crystallized as late, interstitial which represents but a tiny fraction of the total subsurface material. The equivalent rock in the Voisey's Bay area (leop- target area in the PLI (Figure 9). Magmatic sulphide ard troctolite) is interpreted to form by rapid growth of deposits present formidable challenges to exploration, par- clinopyroxene crystals in sulphide-bearing magma, in which ticularly in blind environments, as they are small and lack sulphide is "pushed aside" into interstitial areas (Naldrett et associated alteration haloes. Results to date, although obvi- al., 1996). An alternate origin for this texture is the intro- ously not those that were hoped for, do not entirely diminish duction of sulphides into a partially crystalline magma in the potential of the PLI for economic discoveries. which clinopyroxene and olivine crystals were already forming. Kerr (1998b) suggested that the lower homoge- The evaluation of "potential grade" is a critical step in neous part of the mineralized sequence formed in this way, exploration. Recalculation of assays from sulphide zones to i.e., that sulphides (and fragmental debris) from the com- 100 percent sulphides indicates the "grade ceiling" for asso- posite gabbro unit "rained down" into preexisting, partly ciated massive sulphide mineralization, should it occur. The crystallized, fine-grained olivine gabbro parent magma. The empirical and calculated data from the PLI indicate that latter was cooler and completely solidified near the basal most sulphides contain maximum values of around 2% Ni, contact, resulting in a lower barren gabbro into which sul- slightly less Cu, and around 0.2% Co (Figure 6). These phides did not penetrate. agree well with the better massive sulphide intersections, but many massive sulphide intersections show significantly This remains a possible explanation for the persistent lower absolute grades of 1.0 to 1.2% Ni, which are not fully stratigraphy of the mineralized sequence, and for the unusu- explicable through silicate dilution. In summary, most of the al leopard gabbro texture, but it does not fully explain the present data suggests that potential massive sulphides would observed variation in metal contents (Figure 6). Sulphides in not show the high Ni grades (4% or more) known from the composite gabbro commonly show higher Ni and Cu Voisey's Bay, but would probably have higher Co contents, contents (1.5 to 3.0%) compared to those in the underlying which may in part compensate.

249 CURRENT RESEARCH, REPORT 99-1

Figure 9. Comparisons of the size of the target intrusion at the Voisey's Bay project and the Pants Lake intrusion, as it is cur- rently defined. For comparison, the outcrop area of the Talnakh intrusion in the Norilsk district is also shown. The Ovoid deposit is all but invisible at this scale, located at the head of the arrow (top right).

There are, however, scattered indications of sulphides involves the extraction of a small aliquot from something that are richer in metals, notably in the Happy Face and that is significantly larger. To use a suitable alcoholic analo- Taheke Lake lobes, where Ni and Cu are locally both above gy, a small amount of fine brandy is made (indirectly) from 3% in selected holes. There are also higher Ni and Cu tenors grapes, but a substantially larger quantity of champagne in parts of the South intrusion mineralized sequence. The must first be made as an intermediate step. As indicated by possibility that there were two discrete episodes of mineral- Figure 9, the vineyard is large and by no means fully ization lends uncertainty to the identification of a specific explored at this stage. massive sulphide zone, for some may be connected to an earlier (?) metal-poor episode. Lastly, if there were two episodes of mineralization marked by a change in tenor, CONCLUSIONS could there have been a third with still higher metal values? Continued exploration in the area, university research Although the famous hole 75 intersection shows exact- projects and future activity by the Geological Survey will ly the same Ni/Cu ratio as all other mineralization, other develop and expand, or perhaps disprove entirely, some of higher-grade massive sulphide intersections, with the possi- the early views expressed here. However, this area clearly ble exception of hole 131, have disturbed Ni/Cu ratios demonstrates that potential environments for magmatic sul- (Figure 8) suggesting that they result from fractionation of phide deposits are not restricted to the immediate Voisey's sulphide liquids. Although such rich material is unlikely to Bay area, and the distinctive rock types and processes be widespread, it represents an attractive target in conjunc- observed and inferred at Voisey's Bay prevail in at least one tion with larger, lower grade deposits. Finally, the fractiona- other part of Labrador. Although areally extensive compared tion process is by itself significant, as it by definition to the Reid Brook intrusion, the PLI remains a small and

250 A. KERR inconsequential body at the scale of Labrador, and rocks of synthesis. Canadian Journal of Earth Sciences, Volume this affinity may outcrop or lie in the shallow subsurface in 34, pages 337-351. other parts of this hinterland. Emslie, R.F. 1980: Geology and petrology of the Harp Lake ACKNOWLEDGMENTS Complex, central Labrador: an example of Elsonian magmatism. Geological Survey of Canada, Bulletin This report includes significant direct and indirect con- 293, 136 pages. tributions from Teck Corporation and Donner Minerals staff involved in the 1996 to 1998 exploration programs. In 1996: Troctolitic rocks of the Reid Brook Intrusion, alphabetical order, I wish to thank Kelly Emon, Dennis Nain Plutonic Suite, Voisey Bay area, Labrador. In Fitzpatrick, Steve House, Floyd House, Harvey Keats, Tom Current Research, Part C. Geological Survey of Lane, Heather MacDonald, Guy McGillivray, Paul Moore, Canada, Report 96-1C, pages 183- 196. Trevor Rice, Rod Smith and Kerry Sparkes. The work on which this article is based would not have been possible Evans-Lamswood, D.M., Butt, D.P., Jackson, R.S., Lee, without their efforts, and its contents were improved by their D.V., Muggridge, M.G., Wheeler, R.I. and Wilton, D. input in the field and during preparation. Kerry, Paul and 1998: Physical controls on the distribution of sulphides Harvey contributed directly to the contents of the article in the Voisey's Bay Ni–Cu–Co deposit, Labrador. through reviews and suggestions. I also acknowledge valu- Geological Society of America, 1998 Annual Meeting, able discussions with A.J. Naldrett. Toronto, Ontario, Abstract.

Much of the work discussed in this report is based on Fitzpatrick, D., Moore, P., Mac Gillivray, G., House, S. and information from studies of drill core, and our free access to Emon, K. this (and other) material is greatly appreciated. The camp 1998: Report of work, South Voisey's Bay Project, management staff, particularly Bing Lovang and George Central Labrador: Core program. Teck Explorations Small, are also thanked for their hospitality and assistance. Ltd. Confidential assessment report submitted to the Canadian Helicopters, and particularly pilot Luc Gauthier, Department of Mines and Energy. are thanked for their invaluable services. Joanne Shwetz and John Hinchey are acknowledged for their able, cheerful and Hill, J.D. tolerant assistance to the senior author in 1997 and 1998. 1982: Geology of the Flowers River–Notokwanon Lastly, Carl von Einsiedel (RAM Exploration) is acknowl- River area, Labrador. Newfoundland Department of edged for his work in developing the project in 1995, and for Mines and Energy, Mineral Development Division, encouraging the Geological Survey to become involved in Report 82-6, 137 pages. the project area. Richard Wardle is thanked for reviewing the article. Hodder, S.L. 1997: A drill core analysis and petrographic study of Ni–Cu bearing gabbros, South Voisey's Bay area, REFERENCES Labrador. Unpublished B.Sc. thesis, Memorial University of Newfoundland, St. John's, Amelin, Y., Li, C. and Naldrett, A.J. Newfoundland. 1998: Contrasting assimilation patterns in the Voisey's Bay Complex and Mushua Complex, Labrador, Kerr, A. Canada, indicated by Nd–Pb–Sr isotope systematics. 1998a: Petrology of magmatic sulphide mineralization Geological Society of America, 1998 Annual Meeting, in northern Labrador: preliminary results. In Current Toronto, Ontario, Abstract. Research. Newfoundland Department of Mines and Energy, Geological Survey Branch, Report 98-1, pages 1997: Multistage evolution of the Voisey's Bay 53-75. Complex, Labrador, Canada, revealed by U–Pb systematics of zircon, baddeleyite and apatite. American 1998b: Preliminary report on petrographic studies, Geophysical Union, 1997 Fall Meeting, San Francisco, South Voisey's Bay project area, Labrador. Confidential CA, Abstract. report provided to Donner Minerals and Teck Corporation, April 1998. Barnes, S.J., Zientek, M.L. and Severson, M.J. 1996: Ni, Cu, Au and platinum-group element contents Kerr, A. and Smith, J.L. of sulphides associated with intraplate magmatism: A 1997a: The search for magmatic Ni–Cu–Co mineraliza-

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tion in northern Labrador: A summary of active explo- Ni–Cu–Co Deposit. Geological Society of America, ration programs. In Current Research. Newfoundland 1998 Annual Meeting, Toronto, Ontario, Abstract. Department of Mines and Energy, Geological Survey Branch, Report 97-1, pages 73-93. Ryan, A.B. 1990: Nain–Nutak Compilation Map. Newfoundland 1997b: A summary of 1997 mineral exploration activi- Department of Mines and Energy, Geological Survey ties in the northern Labrador nickel play. Branch, Map 90-44, scale 1:500 000. Newfoundland Department of Mines and Energy, Geological Survey Branch, Report of Activities for 1996: Commentary on the location of the Nain– 1997, pages 21-25. Churchill boundary in the Nain area. In Current Research. Newfoundland Department of Mines and 1998: A summary of 1998 mineral exploration activity Energy, Geological Survey, Report 96-1, pages 109- in northern Labrador. Newfoundland Department of 131. Mines and Energy, Geological Survey Branch, Report of Activities for 1998, pages 16-21. Ryan, B., Wardle, R.J., Gower, C.F. and Nunn, G. 1995: Nickel–copper sulphide mineralization in Li, C., Naldrett, A.J. and Krstic, S. Labrador: The Voisey Bay discovery and its exploration 1996: The Voisey's Bay Ni–Cu–Co deposit: crustal con- implications. In Current Research. Newfoundland tamination and petrological variation. Geological Department of Mines and Energy, Geological Survey, Society of America, 1996 Annual Meeting, Denver, Report 95-1, pages 177-204. Colorado, Abstract. Smith, R.L. and Wilton, D.H.C. Li, C., Amelin, Y. and Naldrett, A.J. 1998: Preliminary investigation of the mineralized and 1998: U–Pb geochronology of the Mushua troctolite contaminated sequence of the Pants Lake Intrusion, complex at Voisey's Bay, northern Labrador, Canada. South Voisey's Bay Project, Labrador. Canadian Geological Society of America, 1998 Annual Meeting, Institute of Mining, Metallurgy and Petroleum, Toronto, Ontario, Abstract. Newfoundland Branch, Annual Meeting, November 1998. Program with Abstracts, page 7. Naldrett, A.J. 1989: Magmatic Sulphide Deposits. Clarendon/Oxford Thomas, A. and Morrison, R.S. University Press, Oxford, United Kingdom. 1991: Geological map of the central part of the Ugjoktok River (NTS 13N/5 and parts of 13M/8 and Naldrett, A.J., Keats, H., Sparkes, K. and Moore, R. 13N/6), Labrador (with accompanying notes). New- 1996: Geology of the Voisey's Bay Ni–Cu–Co deposit, foundland Department of Mines and Energy, Geologi- Labrador, Canada. Exploration and Mining Geology cal Survey Branch, Map 91-160, scale 1:50 000. Journal, Volume 5, pages 169-175. Vernon, R.H. Naldrett, A.J., Li, C., Asif, M. and Amelin, Y. 1984: Microgranitoid enclaves in granites – globules of 1998: The Voisey's Bay Ni–Cu–Co deposit, Labrador, hybrid magma quenched in a plutonic environment. Canada: A model for ore genesis. Geological Society of Nature, Volume 309, no. 5967, pages 438-439 America, 1998 Annual Meeting, Toronto, Ontario, Abstract. Wares, R. 1997: South Voisey's Bay Project: Summary report and Ripley, E.M., Young-Rok, P., Li, C. and Naldrett, A.J. proposed exploration program Donner Resources. 1997: Sulfur and oxygen isotope studies of the Voisey's Confidential assessment report submitted to Bay Ni–Cu–Co deposit, Labrador, Canada. American Department of Mines and Energy. Geophysical Union, 1997 Fall Meeting, San Francisco, CA, Abstract. Weibe, R.A. 1987: Evidence for stratification of basic, silicic and Ripley, E.M., Young-Rok, P., Naldrett, A.J. and Li, C. hybrid magmas in the Newark Island layered intrusion, 1998: Oxygen isotope studies of the Voisey's Bay Nain, Labrador. Geology, Volume 15, pages 349-352.

252 A. KERR

APPENDIX: CALCULATION OF SULPHIDE TENOR

Recalculation of analytical data to 100 percent sulphide because the ultramafic host rocks are richer in olivine and is based on the premise that Ni, Cu and Co are essentially pyroxene. These values are actually higher than the contained in the sulphide phases. Thus, if the sulphide con- observed average values for PLI unmineralized rocks and tent of a mineralized rock is known, the "potential grade" of drill core, and also agree well with the "worst" assay results massive sulphide accumulations can be readily calculated. from rocks with only minute traces of sulphide. The correc- In addition to predictions of economic interest, the method tion method is a simple mass balance calculation, using the provides a consistent method for comparison of rocks with percentages of sulphide and silicate. The conservative val- different sulphide contents from different areas or units. The ues for Ni, Cu and Co in sulphide-free rocks again help to sulphide content is provided by the ratio between elemental ensure that results are not unduly optimistic. For Ni and Cu, sulphur content and the theoretical value for pure pyrrhotite the corrections are trivial, unless the sulphide content is very (FeS; 36.5 weight % S). Here, a slightly lower value of 35 low, but for Co they are larger, because Co is an order of percent sulphur is used, to account for other sulphide miner- magnitude less abundant in the sulphide phase. Thus, 100% als such as chalcopyrite and pentlandite, and to keep esti- sulphide values for Co are very sensitive to correction mates conservative. For example, the sulphide contents and assumptions, and are less reliable. Ni content at 100 percent sulphide are: In assessing data, a negative correlation between calcu- Sulphide Content % = ((weight % S) / 35) x 100 lated Ni and Cu in sulphides and the amount of sulphides is Ni (100 % sulphide) = Ni x ( 35 / weight % S ) commonly observed. There is also significantly greater scat- ter at low sulphide abundances. These have also been noted The method is complicated somewhat by the presence in other studies also (e.g., Barnes et al., 1996), and reflect of Ni, Cu and Co in olivine and, to a lesser extent, pyroxene. several factors. Decreased analytical accuracy for sulphur These force a correction for metals in silicates to be applied, and increased uncertainties for silicate corrections are part- although this is often negligible. There are several methods ly responsible, but there are also real variations that are for correction, some of which involve assumptions about the probably due to local R-factor variations where small percentage of olivine and its Ni content. In this study, a sim- amounts of sulphide were present in isolation. Most of the ple empirical correction is used, by assuming that sulphide- data in Figure 6, aside from some South intrusion material, free gabbro contains 100 ppm each of Ni and Cu and 50 ppm represents samples containing 2 wt % or more sulphur, Co. These values were doubled for the South intrusion data, which translates to minimum sulphide contents of about 6%.

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